Hydrocracking of Heavy Oil by Means of In Situ Prepared

Dec 9, 2013 - The hydrocracking activity of the as-prepared ultradispersed catalyst was evaluated using a semibatch reactor setup under 110 bar of hyd...
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Hydrocracking of Heavy Oil by Means of In Situ Prepared Ultradispersed Nickel Nanocatalyst Salman Alkhaldi and Maen M. Husein* Department of Chemical and Petroleum Engineering, University of Calgary, Calgary, Alberta, Canada T2N 1N4 ABSTRACT: This work adopts a water-in-oil (w/o) microemulsion method for the in situ preparation of ultradispersed metallic nickel (Ni0) nanocatalyst in heavy oil and assesses its hydrocracking activity. Catalyst preparation involved reducing Ni2+ added to the water pools of the heavy oil matrix in the form of aqueous Ni(NO3)2 solution using hydrazine. The volume of the aqueous precursors was limited to values which corresponded to visually stable single heavy oil phase. The product particles were collected by addition of toluene and characterized using XRD, TEM, and EDX. These techniques confirmed the formation of nickel nanoparticles of 22 ± 5 nm mean diameter. The hydrocracking activity of the as-prepared ultradispersed catalyst was evaluated using a semibatch reactor setup under 110 bar of hydrogen and 370 °C. Although no presulfiding was performed, XRD of the spent catalyst confirmed the formation of Ni3S2 nanoparticles with a mean particle size of the same range as the Ni0 particles. Results showed 2-fold improvement in the gaseous fractions, around 47% conversion of the residue, more than 70% reduction in the resins, around 50% reduction in the asphaltenes and an increase in aromatics and saturates in the presence of the ultradispersed catalyst.

1. INTRODUCTION Hydrocracking is part of the oil upgrading process where long chain hydrocarbons are broken into smaller chains in presence of hydrogen. Hydrocracking is a two-stage process combining catalytic cracking and hydrogenation.1 Therefore, hydrocracking catalysts normally contain a cracking function as well as a hydrogenation function. The cracking function is typically provided by an acidic support such as silica−alumina and crystalline zeolite, whereas the hydrogenation function is provided by a metal such as palladium, platinum, etc.2 These metals catalyze the hydrogenation of the feedstock, making it more reactive toward cracking and heteroatom removal as well as reducing the rate of coking.2 The ratio between the catalyst cracking and hydrogenation functions can be adjusted in order to optimize activity and selectivity. Activity, selectivity, stability, and product quality are four key performance measures of hydrocracking catalysts. Ultradispersed (UD) catalysts display a number of characteristic advantages over the supported bulk catalysts, including minor deactivation, high inhibition of coke formation, ease of resulfiding, and a high degree of catalytic metal utilization due to the absence of diffusion limitations.3−6 UD catalysts possess much more accessible reactive sites per unit mass, thereby allowing large complex molecules, including heavy hydrocarbons, to react rather than plugging pores as in supported catalysts.7 For example, dispersed Mo-based catalyst, derived from MoO2 (acetylacetonate)2, increased the conversion of the extra-heavy crude fractions with boiling points >500 °C to lighter cuts in the presence of methane as a source of hydrogen.8 It is worth noting that the heavy oil upgrading activity of a UD catalyst is metal dependent. Mo, Ni, and Febased catalysts gave higher conversion and lower asphaltene content compared with other metals (e.g., Cr, V, and Co).5,9 Galarraga and Pereira-Almao10 employed Ni−W−Mo-based UD catalyst obtained from water-in-oil emulsions for the hydrocracking of Athabasca bitumen and reported an increase © 2013 American Chemical Society

in the hydrogen/carbon ratio and a decrease in viscosity and coke formation. Liu et al.11 prepared MoS2 from highly dispersed Mo precursor in the presence of external source of H2S or sulfur-containing species at 420 °C after 1 h presulfiding period. The resultant catalyst enhanced the radical thermal reactions even under hydrocracking conditions, as was evident from residue conversion and coke and gas yields. Liu et al.12 studied the effect of different presulfiding methods, including breaking the emulsion and colloidal sols, on the reactivity of Nibased catalyst for residue upgrading in slurry-phase hydrocracking. They reported higher reactivity for the nickel sulfide colloidal sol toward hydrogenation and radical-quenching reactions and inhibiting coke formation.12 Luo et al.13 added nonionic surfactant to heavy oil in order to help forming finely dispersed “water-soluble” catalyst which was a blend of NiS and FeS. UD catalyst performance at long residence time was evaluated by testing 1200 ppm trimetallic catalyst in bitumen at 340 °C and 360 psi of H2 for 72 h in a batch reactor. The oil was upgraded from 8 API to 17 API.14 Also, a different catalyst was tested at 1000 ppm under 360 °C and 375 psi of H2 for 16 h and it achieved 50% conversion of the bitumen.14 Water-in-oil (w/o) microemulsions provide ideal media for the preparation of UD nanocatalyst by virtue of their ability to stabilize and limit the size of the resultant particles.15−18 Nassar and Husein19 showed that heavy oil matrixes composed of vacuum residue (VR) and vacuum gas oil (VGO) can stabilize water pools in a similar fashion to w/o microemulsions by virtue of their naturally existing surfactants. Furthermore, they used w/o microemulsion methods to prepare different types of UD nanoparticles in heavy oil matrixes.19,20 Even though metallic catalysts have wide application in heavy oil upgrading,1 Received: August 30, 2013 Revised: December 8, 2013 Published: December 9, 2013 643

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Figure 1. XRD pattern of Ni0 nanoparticles collected from the w/o microemulsion system. Upon mixing of the two microemulsions, the color turned pink while maintaining its transparency. The final system was placed in a shaker at 70 °C and 200 rpm for 12 h. No precipitate or phase separation was observed, and the particles could only be collected for characterization after breaking the microemulsion system with water. For the heavy oil matrix, the same reagent proportions and reaction conditions were employed; however, the single microemulsion technique27 was adopted due to difficulties associated with transferring heavy oil between beakers, which ultimately resulted in loss of reagents. The heavy oil matrix was composed of 20 wt % Athabasca vacuum residue (VR) and 80 wt % Athabasca vacuum gas oil (VGO). The oil was heated to 70 °C in order to reduce its viscosity. Experiments showed that 10 mL of the heavy oil matrix can hold up to 0.6 mL of deionized water without any noticeable phase separation. A 0.3 mL volume of 9.2 M aqueous Ni(NO3)2 precursor was added to 10 mL of the heavy oil matrix, and the matrix was mixed for 5 min in an incubator shaker at 70 °C and 200 rpm. Then, a 0.2 mL aqueous solution containing the hydrazine and the NaOH reagent was added to the same heavy oil matrix, and the system was placed back in the shaker at 70 °C and 200 rpm for 24 h. 2.2. Particle Characterization. The nanoparticles were collected from the w/o microemulsion system by breaking the microemulsion with excess water. The precipitate was collected after centrifuging for 15 min at 500 rpm, washed with water several times, dried at room temperature for 12 h and, then, further dried in an oven at 70 °C for 2 h. Particle recovery from the heavy oil system involved addition of toluene at a volume ratio of 7 mL toluene/1 mL heavy oil. The precipitate was collected after centrifuging for 15 min at 500 rpm and washed several times with toluene (99.8%, VWR, Canada) to remove adsorbed materials. The precipitate was allowed to dry for 12 h at room temperature and then heat treated at 300 °C for 1 h under N2 atmosphere to further remove adsorbed materials. In some experiments, the heat treatment step was avoided in order to minimize nanoparticle aggregation, and the precipitate was directly characterized following drying at room temperature. The spent catalyst was collected from the upgraded oil by centrifugation and washed with dichloromethane (DCM) (anhydrous, ≥ 99.8%, Sigma Aldrich, U.S.A.) to remove adsorbed materials.

to the best of our knowledge, no reports on the preparation of UD metallic nanoparticles in heavy oil exist in the literature. In addition to heavy oil upgrading, dispersed metallic nanoparticles have application in improving combustion efficiency of fuel oils21 and enhancing thermal conductivity, thermal diffusivity, and viscosity of fluids used in industrial and engineering applications.22 In this work, and following a procedure proposed by Chen and Wu,23 for Ni0 nanoparticle preparation in (w/o) microemulsions, UD Ni0 nanoparticles were prepared in heavy oil composed of 20 wt % VR and 80 wt % VGO from Athabasca. The hydrocracking activity of the as-prepared catalyst was assessed in a semibatch reactor setup and the properties of the upgraded samples were compared with the original feed and control samples.

2. EXPERIMENTAL METHODS 2.1. Nanoparticle Preparation. The (w/o) microemulsion system was formed by addition of 3.7 g of cetyltrimethylammonium bromide, CTAB (98%, Alfa Aesar, Toronto, ON) along with 3.7 g 1butanol (99%, Sigma-Aldrich Fine Chemical, Toronto, ON) to hexan (98%, VWR, Mississauga, ON) in a 200 mL volumetric flask. The mole ratio of 1-butanol to CTAB, P0, was fixed at 5.5, while the molar ratio of water to surfactant, R, was fixed at 2. The preparation of Ni0 nanoparticles followed the mixing of two microemulsions technique, where one microemulsion contained the nickel salt and the other contained the reducing agent. A 0.4 mL volume of 4.3 M aqueous Ni(NO3)2 (puratronic grade, Alfa Aesar, Toronto, ON) solution was added to the first microemulsion and the required amounts of N2H4 (98% Sigma-Aldrich, Toronto, ON) and NaOH (Sigma-Aldrich, Toronto, ON) were added to the second microemulsion. The molar ratio of N2H4 to Ni(NO3)2 used in this experiment was 1323−25 and the molar ratio of NaOH to Ni(NO3)2 was 2 as suggested by reaction R1 below.23,26

2Ni 2 + + N2H4 + 4OH− → 2Ni + N2 + 4H 2O

(R1) 644

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Figure 2. (a) XRD pattern; (b) TEM photograph; and (c) EDX spectrum for Ni0 particles collected from the heavy oil matrix. 645

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Figure 3. XRD pattern of the spent catalyst particles. mL/min into the pyrolysis tube held at temperature 1050 °C and the O2 flow were 300 mL/min. Following Galarraga and Pereira-Almao,10 the conversion of the residue, 545+ °C, for all samples was calculated using the eq E1:

The precipitates were ground using a pestle and mortar and introduced to Ultima III Multi-Purpose Diffraction System (Rigaku Corp., The Woodlands, TX) for XRD analysis. Part of the dried precipitate was used for TEM analysis. Details on sample preparation and settings for the XRD and TEM instruments can be found elsewhere.28 2.3. Catalyst Performance. After confirming the formation of Ni0 nanoparticles in heavy oil, the performance of the as-prepared catalyst was tested using a semibatch reactor setup (4590Micro Bench top Reactor, Parr Instrument Company, Moline, IL). A 50 mL volume of the heavy oil containing 4000 ppm of the in situ prepared ultradispersed Ni0 nanocatalyst was placed in the reactor. For upgrading, H2(g) was allowed into the system using a sparger located at the bottom of the reactor vessel and the vessel was pressurized to 105 bar. The reactor was then heated to 370 °C under continuous mixing at 160 rpm for 24 h. The above pressure and temperature are typical for lab scale upgrading.7,29,30 A representative sample was collected from the upgraded oil, following effective mixing, for analysis. The effect of heat treatment, H2 bubbling, and H2O and N2H4 addition were studied by running control experiments without nanoparticles. The control experiments were subjected to the same temperature and pressure for the same time duration. Representative samples were also collected from the control experiments for analysis. One more control sample was provided in order to differentiate between thermal and hydrocracking, which involved heating a heavy oil matrix containing 4000 ppm dispersed Ni0 nanoparticles at 370 °C for the same time period, however, without H2 bubbling in a batch system. Simulated distillation (SimDist) results were collected on an Agilent 6890N gas chromatograph (Agilent Technologies Canada Inc., Mississauga, ON) following ASTM D-7169 procedure as detailed by Carbognani et al.31 Mass loss from the semibatch reactor setup was determined by measuring the initial and final volumes and densities (digital density meter, DMA 45 Anton Paar, Austria) of the samples. The density measurements (digital density meter, DMA 45 Anton Paar, Austria) were also used to calculate API gravity of the samples. Saturates, aromatics, resins, and asphaltenes (SARA) analysis was performed by coupling microdeasphalting with thin layer chromatography as detailed by Carbognani et al.32 Total sulfur content in the feed, control, and product samples was measured using Trace Sulfur Analyzer Model TS-100 (Mitsubishi Chemical Analytech Co. LTD, Japan). The sample was injected with Argon carrier gas at flow 400

Conv545+ °C (%) =

Mass of 545+ °Cfeed − Mass of 545+ °Cproduct Mass of 545+ °Cfeed × 100

(E1)

3. RESULTS AND DISCUSSION 3.1. Microemulsion-Prepared Ni0 Nanoparticles. Figure 1 shows the X-ray diffraction pattern of the particles collected from the w/o microemulsion system. A comparison with JADE software confirms the presence of all major peaks belonging to nickel (2θ = 44.5°, 51.8°, and 76.4°) (JCPDS card No. 01-0713740). In addition, peaks belonging to CTAB existed at low θ 33, 34. As reported by other researchers,23 washing the particles several times with water or organic solvents could not completely remove the adsorbed CTAB. A mean crystalline size calculated from Scherrer’s equation based on the peak at 2θ = 44.6° was 20 nm. 3.2. Oil Matrix-Prepared Ni0 Nanoparticles. Following the successful preparation of Ni nanoparticles in w/o microemulsions, similar procedure was adopted for preparing these particles in the heavy oil matrix as detailed in the Experimental Section. The XRD (JCPDS card No. 01-0713740), TEM, and EDX results for particles collected from the heavy oil matrix and heat treated at 300 °C under N2 atmosphere to removed adsorbed organics are shown in Figure 2a−c. It is clear from the figure that nickel nanoparticles were successfully formed in the heavy oil matrix, and heat treatment under nitrogen did not seem to affect their identity.33−35 The mean crystalline size calculated by applying Scherrer’s equation from the XRD peak at 2θ = 44.6° was 17 nm. The mean particle diameter calculated from the TEM images of 40 particles was 22 ± 5 nm, which is not very different from the XRD estimate. The EDX analysis in Figure 2c further confirms the existence of nickel in the particles. The presence of Cu in 646

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Figure 4. SimDist results for different samples involved in this study: ( × ) original feed; (▲) hydrocracked sample in the presence of the UD nanocatalyst; (◇) control sample without additives; (□) control sample containing H2O only; (○) control sample containing N2H4 only; (●) thermallycracked sample in presence of the UD nanocatalyst. 95% C.I. is included for samples □ and ▲.

Table 1. Percent Mass Loss from the Semi-Batch Reactor Setup, Total Sulfur Content, API Gravity, Mass Fraction of the Residue and Percent Conversion of the Residue for the (1) Original Feed, (2) Hydrocracked Sample in Presence of the UD nanocatalyst, (3) Control Sample without Additives, (4) Control Sample Containing H2O Only, (5) Control Sample Containing N2H4(aq) Only sample 1 2 3 4 5

mass loss (%)

total sulfur (wt %)

API gravity

Residue545+°C (wt %)

Conv545+°C (%)

19.3 ± 3.0 10.9 7.5 9.7

3.4 2.7 ± 0.2 2.8 2.9 2.4

14.9 17.1 ± 1.7 19.6 17.2 17.7

23 12.2 ± 1.4 17.0 17.0 14.0

47.0 ± 6.0 26.0 26.0 39.0

display similar trend as the control samples and differences between the boiling points reasonably fall within the 95% confidence interval. Nevertheless, this result should be discussed in conjunction with the mass loss data from the semibatch reactor system presented in Table 1. The table shows that, despite the SimDist results, samples subjected to upgrading in the presence of dispersed nanoparticles displayed at least 2-fold more mass loss than the rest of the samples, including the one with hydrazine, and accounted for approximately 10% more conversion into gaseous fractions. Given the critical temperatures of hydrocarbons39 and the reactor temperature and pressure, it is believed that the samples containing the nanoparticles produced more upgrading; however, most of the upgraded oil, in fact, left the semibatch reactor setup as vapor. Measuring the contribution of the upgraded gaseous products to the total stream leaving the semibatch reactor was, however, not straightforward at the relatively high volume of injected H2. It is worth noting that Tanabe and Gray40 also determined gas yield by weighing a batch reactor before and after venting the gas. In order to ensure that this upgrading did not result from enhanced thermalcracking in the presence of the nanoparticles, a thermalcracking experiment was performed for heavy oil containing 4000 ppm Ni0 nanoparticles at 370 °C for 24 h under the system’s natural pressure. The SimDist result for this

this analysis is attributed to the TEM copper grid, while the presence of other elements is attributed to impurities originated from adsorbed organics from the oil. The XRD pattern of the spent catalyst is given in Figure 3. The pattern belongs to Ni3S2, Heazlewoodite, as confirmed by the JADE program (JPCDS card No. 01-085-1802). The source of sulfur is believed to be sulfur-containing species in the heavy oil matrix,11 which resulted in a form of nickel sulfide differrent from those commonly reported as major products from catalyst presulfiding.36−38 The mean particle size of the spent catalyst calculated from Scherrer’s equation at 2θ = 31.2 was 23 nm. 3.3. Performance of the Nanocatalyst. The SimDist results for the samples with and without ultradispersed catalyst subjected to the upgrading conditions of 370 °C and 105 bar H2 for 24 h are plotted in Figure 4. In addition, Figure 4 contains the SimDist results for the original oil matrix. The difference in the boiling points between the original heavy oil matrix and the samples subjected to upgrading suggests that some upgrading took place in the absence of the Ni0 nanoparticles. The performance of the UD nanocatalyst can be seen upon comparing the control samples, that is, the oil matrix, the oil matrix with H2O, and the oil matrix containing hydrazine, with the sample containing the Ni0 UD catalyst all subjected to the same heat treatment and hydrogen pressure. The upgraded samples in the presence of the nanoparticles 647

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the API gravity in the presence and absence of the UD catalyst falls within the 95% confidence interval. Liu et al.11 reported that noncatalytic thermalcracking produces light fraction and more cracking conversion than hydrocracking in presence of catalyst when the sole role of the catalyst is to promote hydrogenation. This said, one should not overlook the mass lost from the semibatch reactor, which was higher in the presence of the UD catalyst, and its impact in lowering the API gravity. The conversion of the residue, 545+ °C, calculated using (E1) and given in Table 1, shows that the upgraded samples in the presence of the UD nanoparticles display the highest conversion of the residue. It should be noted that (E1) accounts for mass loss due to gas formation. The thermally cracked samples in the presence of the UD nanoparticle, on the other hand, displayed lower mass fraction of the residue compared to the original feed. The residue fraction of this sample decreased to 12.2 ± 1.4 wt % compared with 23 wt % of the original feed. Lastly, using dispersed NiS and FeS catalyst, Luo et al.13 observed a variation in catalyst reactivity with mixing and reported an increase in atmospheric gas oil and vacuum gas coupled with a decrease in coke and gas fraction yields at higher mixing rate. The effect of mixing vanished at high mixing rates, it should be noted, however, that the dispersed catalyst employed in Luo et al.’s work was in the range of 1.5 μm. Given the size of our starting and spent catalyst and the fact that hydrogen was introduced to the semibatch reactor through a sparger, we do not anticipate a major role for mixing in the current study.

sample given in Figure 4 shows some upgrading relative to the original oil sample, while heavier fractions were always obtained when compared with any of the control samples. Saturates, aromatics, resins, and asphaltenes (SARA) analysis presented in Table 2 shows an increase in the saturates and Table 2. SARA Analysis Results for the (1) Original Feed, (2) Hydrocracked Sample in Presence of UD Catalyst, and (5) Control Sample Containing N2H4(aq) Only sample

saturates (wt %)

aromatics (wt %)

resins (wt %)

asphaltenes (wt %)

1 2 5

29.2 30.6 32.7

47.5 61.7 58.6

20.1 6.0 6.9

3.2 1.8 1.8

aromatics fractions coupled with a significant decrease in the resins and asphaltenes, which are precursors for coke formation, in presence of the UD nanocatalyst and the N2H4(aq) precursor. While, both the UD nanocatalyst and the hydrazine precursor reduced asphaltenes and resins content to the same extent, it appears that the hydrazine precursor converted more of the aromatics into saturates. However, this observation should not be separated from the fact that production of gaseous fractions was higher in the presence of the UD nanocatalyst. In general, increasing the saturates and aromatics fractions may result from creating H● radicals, which help terminating the long chain radical molecules and inhibit the condensation reaction of long chain radicals.11,13,36 The fact that the UD catalyst employed in this work did not have acidic function led to thermally controlled C−C bond cleavage reactions38,41,42 followed by hydrogenation reactions by means of H● radicals supplied by the UD catalyst. Consequently, we believe the current upgrading reactions followed the free radical mechanism. The ability of the UD nanocatalyst to remove sulfur from the heavy oil was assessed by measuring the total sulfur content in the original feed, upgraded oil in presence of the UD catalyst and the control samples. The results shown in Table 1 confirm that there was a reduction in the sulfur content in the upgraded oil in the presence and absence of the UD nanocatalyst. With the exception of the sample containing the N2H4(aq) precursor, the difference in the sulfur content in the presence and absence of the UD catalyst falls within the 95% confidence interval and sulfur removal was limited to ca. 20%. This suggests that the reduction in the sulfur is not related to a catalytic effect, despite the fact that the Ni0 catalyst was converted to Ni3S2. The most common hydrosulfrization (HDS) catalysts are the transition metal sulfides such as Co9S8, Ni3S2, MoS2, and WS2.43 The transition metal sulfides are active toward hydrogenation reaction,44 which is a key reaction during HDS.45 In the HDS process, sulfur atoms are removed following hydrogenating the reactant to an intermediate state, where the C−S bond is broken.46 On the other hand, Aray et al.47 reported that Ni3S2 nanoparticles are inactive toward HDS because of the low attraction between the unsaturated active sites (Lewis sites) of the catalytic particles and the S atoms coming from the feed. Using theoretical calculations, they showed that that unsupported Ni3S2 nanoparticles under HDS conditions expose (111) and (111̅), which has low Lewis acidity. Gravity measurements reported in Table 1 reflect an increase in the API gravity of the upgraded oil in the presence and absence of the UD nanocatalyst. With the exception of the upgraded sample without precursor or H2O, the difference in

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CONCLUSION Ultradispersed metallic nickel nanoparticles were successfully prepared in heavy oil starting from Ni(NO3)2 aqueous precursor and using hydrazine as the reducing agent employing w/o microemulsion methods. XRD patterns for particles collected from the w/o microemulsion and heavy oil systems confirmed the formation of the metallic nickel nanoparticles. It is believed that naturally occurring surface active agents in the heavy oil acted as surfactants and helped stabilizing the resultant particles and limiting their size to around 22 ± 5 nm, as per the TEM photographs. Following upgrading, the spent catalyst was converted to Ni3S2, with similar particle size, probably due to reactions with sulfur-containing species in the heavy oil. The catalyst promoted hydrogenation and free radical reactions and resulted in 50% reduction in asphaltenes and 70% reduction in resins, while improved the aromatics and saturates fractions. API gravity and sulfur removal were comparable for the samples containing the UD nanocatalyst and the control sample. Nevertheless, the role of the UD catalyst is evident, relative to control samples, when one accounts for the mass loss due to gas formation during the reaction. 2-fold increase in the gaseous fraction and around 47% conversion of the residue, conv545+oC, occurred in the presence of the UD catalyst.

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AUTHOR INFORMATION

Corresponding Author

*Phone 403-220-6691. Fax (403) 282-3945. E-mail: maen. [email protected]. Notes

The authors declare no competing financial interest. 648

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the Natural Sciences and Engineering Research Council of Canada (NSERC) and the financial support of Saudi Aramco in the form of a scholarship awarded to Mr. Alkhaldi. The help of Dr. Francisco Lopez-Linares and Mr. Lante Carbognani with the SimDist and SARA analyses and the help of the Alberta Sulphur Research Ltd. with sulfur analysis are greatly appreciated.

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