Article pubs.acs.org/EF
Comparison of MoS2 Catalysts Prepared from Mo-Micelle and MoOctoate Precursors for Hydroconversion of Cold Lake Vacuum Residue: Catalyst Activity, Coke Properties and Catalyst Recycle Hooman Rezaei, Shahrzad Jooya Ardakani, and Kevin J. Smith* Department of Chemical and Biological Engineering, University of British Columbia, Vancouver, BC, Canada ABSTRACT: The hydroconversion of cold lake vacuum residue (CLVR) in a semibatch, slurry-phase reactor was studied at 415−445 °C, 13.8 MPa, and a reaction time of 1 h, using MoS2 catalysts prepared from Mo-octoate and Mo-micelle precursors. Both precursors yielded MoS2 with the same activity in terms of coke suppression, residue conversion, and hydrogen uptake. The coke yield decreased from 22 wt % in the absence of a catalyst, to 4.8 wt % in the presence of 100 ppm Mo. A Mo concentration of 600 ppm was found to be optimum in terms of maximizing the residue conversion (84 wt %) and minimizing the coke yield (2.9 wt %). The characteristics of the recovered coke as a function of catalyst concentration and age in the reactor were also investigated. At high catalyst concentrations (>600 ppm Mo) and short reaction times, the generated coke was relatively amorphous, with a high H/C ratio. The solid catalyst−coke recovered from the reactor was recycled without further treatment. The catalytic activity of the recycled catalyst was the same as the fresh catalyst, and no catalyst deactivation was observed under the hydroconversion conditions of the present study.
1. INTRODUCTION The goal of residue oil upgrading is to increase the H/C atom ratio and the API gravity of the product oil, while also producing lower boiling point distillates with higher commercial value than the residue oil feedstock.1 Two categories of residue oil upgrading processes have been developed to achieve this goal: (1) carbon rejection (or coking) processes that decrease carbon content of the feed and (2) hydrogen addition (or hydroconversion) processes that increase hydrogen content of the feed. An essential difference between coking (thermal decomposition in the absence of hydrogen and catalyst) and hydroconversion (mainly hydrogenolysis reactions in the presence of hydrogen and catalyst) is that in coking the production of light products is always associated with the production of polymerized heavier products, such as coke. The polymerization reactions may be partially or even entirely prevented during hydroconversion which usually results in increased distillate yield.2 Hydroconversion processes may be classified according to the fixed-bed, ebullating-bed, or slurry-phase reactor used in the process. At present, most commercial hydroconversion processes use conventional fixed-bed technologies, which are limited by the choice of feedstock and the severity of operation because carbonaceous and metal deposits decrease the activity of the catalyst over extended periods of operation. Ebullating bed reactor technology3 can process heavy feeds at high severity and the catalyst can be added and withdrawn without shutting down the reactor. Despite the advantage of this technology, it is mechanically complex and uses large amounts of expensive catalysts, the disposal of which represents an environmental and economic concern. The slurry-phase hydroconversion technologies that use highly dispersed, unsupported catalysts were developed to address these issues. Since the dispersed catalysts operate “once-through” the bubble-column slurry reactor, deactivation issues are signifi© 2012 American Chemical Society
cantly reduced compared to supported catalysts. Dispersed catalysts can be introduced to the oil as finely divided inorganic powders, water-soluble salts or as oil-soluble salts.4 The presence of a well dispersed catalyst favors the rapid up-take of hydrogen, suppressing coke formation and enhancing the conversion of residue oil into light products. The development of slurry phase catalysts, their hydroconversion mechanisms, and the main processes close to commercialization have been reviewed3,5−9 and several slurryphase hydroconversion processes aimed at residue oil upgrading have been developed over the last few decades. Examples include the M-coke process developed by Exxon in 1981,10 the CANMET process,11−15 the EST process (Eni Slurry Technology),16−19 VCC (VEBA Combi-Cracking),20−22 SOC (Super Oil Cracking),23 Intevep HDH Process24,25 and the (HC)3 process.26,27 The difference between these processes is mostly in the type of catalyst used. Numerous studies have reported on the performance of the catalysts, both in terms of the active metal and in terms of the form of the catalyst added to the slurry-phase reactor.28−47 The performance of different metal sulfides, introduced to the reactor in the form of oilsoluble precursors, has also been reported.31,48−50 The oilsoluble catalysts have superior activity compared to the activity of the catalysts added to the reactor as water-soluble salts or finely divided powders. For example, Sato et al.31 compared an ultrafine MoS2 with MoS2 prepared in situ from oil-soluble Modithiocarbamate during hydroprocessing of Kuwait atmospheric residue (AR). The authors showed that in order to achieve an activity close to the activity of the in situ prepared catalyst, an extensive micronization of the MoS2 was required. Unlike the oil-soluble precursors, water-soluble precursors have captured Received: January 6, 2012 Revised: April 19, 2012 Published: April 24, 2012 2768
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metal, although under reaction conditions the Mo was converted to MoS2. A 250 mL stirred semibatch reactor was used to assess the catalytic activity of the two types of catalyst precursors under high residue conversion conditions. For the experiments reported herein, reaction temperatures of 415, 430, and 445 °C, a H2 gas pressure of 13.8 MPa, a H2 flow rate of 900 mL(STP)/min, and a reaction time of 0 h (referenced as a heat-up experiment) and 1 h were investigated. Note that a reaction time of 0 h corresponded to experiments in which the reaction was immediately quenched once the reactor had reached the desired reaction temperature and the products were recovered and analyzed according to the normal product workup procedure. The quenching was done at high pressure to minimize coke formation during the cool down and depressurization periods.59 Several noncatalytic experiments were also completed in which the reaction was done under the same conditions as the catalytic experiments, except that no catalyst precursor was added to the residue oil. In all experiments, about 80 g of cold lake vacuum residue oil (CLVR, provided by Imperial Oil) was used as feed. Detailed properties of the CLVR, with 74.8 wt % pitch (BP > 524 °C), 6.0 wt % S, and 17.7 wt % asphaltenes, have been reported previously.58 The solid and liquid products of the reaction were recovered and separated using a high speed centrifuge. The recovered solid was washed with toluene to remove any toluene-soluble material. Coke was defined as toluene-insoluble solids in all experiments. The toluene-insoluble content of the liquid recovered from the centrifuged samples was less than 0.5 wt %. The exit gas flow from the reactor passed through a condenser held at 0 °C and a scrubber before being analyzed. The gas analysis data were used to calculate the H2 uptake, and the H2S and total gas yields. Further details on the semibatch reactor used in the present study, as well as the experimental and product workup procedures, have been reported previously.58 The C, H, N, and S analysis of the liquid product was done using a 2400 Perkin-Elmer CHNS/O analyzer. The boiling point distribution of the liquid product was determined by high temperature simulated distillation (HTSD) using the ASTM D7169 method. The HTSD data were used to calculate the toluene insoluble organic residue (TIOR) conversion, defined as
much less interest because of the problems associated with introducing aqueous solutions to the residue oil.4 Fast evaporation of water and agglomeration of precursor salts results in the production of large catalyst particles with low activity. The mechanism of coke suppression in catalytic residue upgrading has been well studied.51−55 Although conversion and cracking of macromolecules and polynuclear aromatic hydrocarbons (PAHs) to smaller molecules is thermally driven, the catalyst transfers hydrogen from the vapor phase to the liquid mixture. While the hydrogen transfer by catalysts to the heavy components has been recognized as an important step, the mechanism of this phenomenon is not well established. Although some studies propose that hydrogen atoms from catalytic centers hydrogenate radicals generated by homogeneous cracking reactions,41 there is no experimental evidence supporting significant selectivity of this method of hydrogen transfer compared to alternative mechanisms.51 The two important roles of the catalyst in upgrading of residue are the hydrogenation of olefins, mainly produced by β-scission of freeradicals56 and the partial hydrogenation of PAHs which have acted as hydrogen donors and become dehydrogenated.51 Due to the complexity of the molecules in residue oils, such catalytic hydrogen transfer reactions can potentially be promoted by solids present in the feedstock and by the metal walls of the reactor vessels. The latter has a noticeable catalytic activity in small-scale experiments.57 In previous work,58 the present authors showed that the catalytic activity of a water-soluble precursor (ammonium heptamolybdate) could be significantly increased by transforming the water-soluble precursor into an oil-soluble precursor using a reversed micelle. The state of the catalyst in the catalyst−coke mixture recovered from hydroconversion reactions was also investigated to determine the potential for catalyst−coke recycle. Results of that study indicated that the recovered catalyst−coke could potentially be reused in subsequent hydroconversion reactions. In the present study, the performance of the MoS2 catalyst prepared from the reversed micelle precursor is compared to that of MoS2 prepared from an oil-soluble precursor. Mo-octoate was chosen as the precursor since it represents a low-cost, oil-soluble alternative to the Mo-micelle precursor. Since the generated coke acts as a support for the MoS2 particles in the recovered coke−catalyst, an understanding of the properties of the generated coke and changes in its properties with catalyst type and concentration, as well as with operating conditions, is necessary. Coke−catalyst characterization results are presented and differences in the coke properties are linked to the catalyst type, concentration, and operating conditions in the hydroconversion experiments. The recovered coke−catalyst was also recycled, and the activity of the recycled coke−catalyst under high residue conversion conditions is compared to the fresh catalysts.
TIOR conversion =
(WR,F − (WR,L + WC)) WR,F
× 100 (1)
where WR,F is the weight of residue (BP > 524 °C) in the feed, WR,L is the weight of residue in the liquid product, and WC is the weight of solid coke recovered from the experiment. In the present work, TIOR conversion is reported since then the coke produced is correctly considered as part of the residue product. The solid coke−catalyst recovered after each hydroconversion reaction was characterized by elemental (C, H, N, and S) and metals analyses, X-ray diffraction (XRD), energy dispersive X-ray (EDX) analysis, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), diffuse-reflectance infrared Fourier transform (DRIFT) spectroscopy, BET analysis, 13C NMR, and high resolution transmission electron microscopy (HRTEM). Elemental analysis was done using a 2400 Perkin-Elmer CHNS/O analyzer. Approximately 1.5−2.5 mg of coke was used for the elemental analysis and combustion gases from the combustion tube were detected using a thermal conductivity detector (TCD). Details of sample preparation and instrumentation used for EDX, HRTEM, and XRD characterization have been reported previously.58 The BET surface areas of the recovered coke−catalyst were determined from N2 adsorption− desorption isotherms measured at 77 K using a Micromeritics ASAP 2020 analyzer. Samples were degassed at 523 K under vacuum (500 μm Hg) for 8 h before being analyzed. Eight N2 uptake measurements, made in the relative pressure range of 0.06−0.20, were used to calculate the BET surface area. Solid state NMR was used to measure the aromatic and aliphatic carbons (CAr and CAl) in the recovered coke. 13C cross-polarization magic angle sample spinning (13C CPMAS) spectra were recorded on a VarianInova 400, as well as a BrukerAvance 500 MHz, instrument operated at 13C frequencies of
2. EXPERIMENTAL SECTION The materials and methods used to prepare the Mo metal in reversed micelles has been described previously.58 In the present study, Momicelles were used at concentrations of 100, 300, 600, and 1800 ppm Mo in the residue oil. Comparative tests were done at the same Mo concentrations using the oil-soluble molybdenum-octoate (15.3 wt % Mo as molybdenum-2-ethylhexanoate provided by Shepherd Chemical Company) catalyst precursor, added directly to the residue oil. Note that throughout we report the catalyst concentrations in terms of Mo 2769
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100.521 MHz and 125.691 MHz, respectively. All samples were spun at 9000 ± 0.002 kHz. XPS analysis was done using a Leybold Max200 X-ray photoelectron spectrometer with an Al Kα X-ray source. TGA analysis was done under N2 gas at a flow of 50 mL (STP)/min using a TGA-50 thermogravimetric analyzer (Shimadzu, Japan) and 5−15 mg of coke. The coke was heated to 120 °C at a rate of 5 °C/min and held for 60 min to remove moisture, followed by heating to 900 °C at a rate of 5 °C/min; the final temperature being held for a further 120 min. DRIFTS analysis of the coke was carried out using a Nicolet 4700 FTIR unit (Thermo Electron Corporation) at ambient temperature. The coke was diluted and dispersed in KBr powder prior to DRIFTS analysis.
genation reactions cause a noticeable increase in the hydrogen uptake, even when the catalyst is added to the reactor in small concentrations, as shown in Figure 2. In the noncatalytic
3. RESULTS AND DISCUSSION 3.1. Effect of Catalyst Precursor and Catalyst Concentration. In the present study, results of experiments in which catalyst precursors were added to the reactor are compared to noncatalytic experiments. Thus, possible effects of the reactor wall are minimized. Furthermore, to ensure that sufficient hydrogen was available during reaction even at high residue conversions, a semibatch reactor with continuous H2 gas flow was used. The H2 flow removes volatile compounds present in the reactor. This increases the viscosity of the remaining liquid and decreases the asphaltene solubility, promoting liquid−liquid phase separation which eventually leads to coke formation.53,59 Consequently, the coke yields reported herein are likely higher than what would be expected from a continuous industrial hydroconversion reactor operating at the same severity. Figure 1 reports the coke and liquid yield as a function of Mo concentration for both the Mo-micelle and Mo-octoate catalyst
Figure 2. Hydrogen uptake during CLVR hydroconversion as a function of Mo concentration for Mo-micelle and Mo-octoate catalyst precursors. Operating conditions: T = 445 °C, PH2 = 13.8 MPa at 900 mL(STP)/min, and 1 h reaction time.
experiment, despite the presence of a high hydrogen pressure in the reactor, the hydrogen was not effectively transferred to the liquid phase. Consequently the olefins and free radicals undergo polymerization and termination reactions and produce a significant amount of an insoluble aromatic-rich liquid phase which eventually produces coke. For both catalyst precursors, an increase in Mo concentration from 100 to 300 ppm decreased the coke yield from 4.8 to 1.9 wt %. Further increases in the catalyst concentration to 600 ppm Mo had no significant effect on the coke yield. When the Mo concentration increased to 1800 ppm, the coke yield (2.2 wt %) increased marginally compared to the coke yield with 600 ppm Mo (1.8 wt %) for both the Mo-micelle and Mooctoate catalyst precursors. These small changes in coke yield with Mo concentration could be a result of two competing mechanisms. On the one hand, the hydrogenation rate is dependent on catalyst concentration and dispersion, while on the other, the catalyst particles (similar to other solid particles in the reactor) may act as nucleation sites for the agglomeration of small solid coke particles at the reaction conditions.4 Consequently, there is a maximum catalyst concentration that minimizes coke yield and in the present study, 600 ppm Mo appears to be the optimum concentration in terms of coke suppression. Similar observations have been reported by others.10,49,60 Panariti et al.60 showed that the competing effects occur over a wide range of hydroconversion temperatures from 400 to 440 °C. In addition to the mechanism described above, Panariti et al.60 claimed that excess hydrogenation of the liquid feed that occurs at high Mo concentrations, reduces asphaltene stability, which eventually causes more coke formation. This mechanism has also been proposed by others.53,61 The goal of hydroconversion is to maximize liquid yield and minimize coke and gas yield at high residue conversions. Figure 1 shows that the liquid yield increased in the catalytic experiments compared to the noncatalytic experiment. In the
Figure 1. Coke and liquid yields from CLVR hydroconversion as a function of Mo concentration for Mo-micelle and Mo-octoate catalyst precursors. Operating conditions: T = 445 °C, PH2 = 13.8 MPa at 900 mL(STP)/min, and 1 h reaction time.
precursors. The coke yield decreased from 21.2 wt % in the noncatalytic experiment to 4.8 wt % with 100 ppm Mo using the Mo-micelle precursor. Clearly the decrease in coke formation is a consequence of the MoS2 catalyst. The coke suppression is due to direct olefin hydrogenation, with the olefins produced by β-scission and partial hydrogenation of PAHs to form hydrogen donor species that prevent polymerization and condensation reactions.51 These catalytic hydro2770
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noncatalytic experiment, the liquid yield was very low (59.0 wt %) due to the high gas (9.1 wt %) and coke (21.2 wt %) yields. By increasing the catalyst concentration, the liquid yield increased significantly to ∼82 wt % with 100 ppm Mo and to ∼88.0 wt % with 300 ppm Mo for both the Mo-micelle and Mo-octoate precursors. A very slow increase in liquid yield with further increases in Mo concentration to 1800 ppm was observed, consistent with the trend observed for the coke yield. Figure 1 also shows that the Mo-octoate precursor had almost the same activity as the Mo-micelle precursor in terms of coke and liquid yields. This indicated that although preparation of an oil-soluble Mo-micelle precursor from a water-soluble Mo salt increased the catalytic activity of MoS2 compared to the activity of the water-soluble precursor alone,58 the Mo-micelle precursor had the same performance as the oilsoluble Mo-octoate precursor. The small differences in liquid yield from the Mo-micelle and Mo-octoate catalyst precursors is believed to be due to the presence of n-hexane used as solvent in the preparation of the Mo-micelle precursor. A fraction of the n-hexane was recovered in the condenser after reaction and this increased the liquid yield in experiments in which the Mo-micelle precursor was used. The hydrogen uptake (Figure 2) also showed a significant increase when MoS2 was added to the reactor and the hydrogen uptake for both the Mo-micelle and Mo-octoate catalyst precursors was almost the same. However, unlike the coke and liquid yields, the hydrogen uptake increased continuously with Mo concentration up to 1800 ppm, suggesting that at high Mo concentrations (600 and 1800 ppm) although the hydrogen was being consumed, it was not being effectively transferred to the unstable, cracked molecules to suppress coke formation. According to the chain reaction mechanism for liquid-phase cracking proposed by Gray and McCaffrey,51 which is an extended form of the LaMarca−Libanti−Klein−Cronauer (LLKC) free-radical chain reaction mechanism,56 the olefin and aromatic hydrogenation reactions are catalytic. Hence, the high catalyst concentration yields more hydrogenated product. The continued increase in H 2 consumption with Mo concentration, reported in the present study, also suggests that no matter how fast the rate of hydrogen transfer from the gas phase to the free radicals and PAHs, polymerization and condensation reactions occur and cannot be completely prevented. Hence some products have significantly higher aromatic content than the feed, which eventually causes liquid phase separation and coke formation.55 Once phase separation occurs, the formation of toluene-insoluble material (coke) is extremely fast.55 Consequently, zero coke formation during hydroconversion of residue oil under hydroconversion conditions which yields significant residue conversion has never been reported. In summary, the hydrogen consumption that occurred with increased catalyst concentration resulted in the hydrogenation of olefins and aromatics and the production of more hydrogenated products (Figure 3), not additional coke suppression. This result is consistent with a marginally increased coke yield due to excess hydrogenation of the liquid phase and instability of the asphaltene phase in the reactor when a very high Mo concentration (1800 ppm Mo) was used (Figure 1). Although the hydrogen uptake increased when the Mo concentration increased from 600 to 1800 ppm, the rate of this increase was much lower than when the Mo concentration increased from 0 to 600 ppm. This could be explained by the fact that there are potentially two sources of hydrogen
Figure 3. H/C atom ratio and CAr/CAl ratio of solid (coke) and liquid products from CLVR hydroconversion: (●) H/C atom ratio of liquid products, (■) H/C atom ratio of coke, (◊) H/C atom ratio of coke, and (▲) CAr/CAl ratio of coke as a function of Mo concentration for Mo-micelle and Mo-octoate catalyst precursors. Operating conditions: T = 445 °C, PH2 = 13.8 MPa at 900 mL(STP)/min, and 1 h reaction time.
consumption in hydroconversion reactions. The first is hydrogen transfer by hydrogen donor compounds to the free radicals generated during cracking, which is likely responsible for most of the hydrogen consumption since the numerous side chains of the complex molecules present in residue oil would generate a substantial number of free radicals. The second source of hydrogen consumption is hydrogenation of compounds with unsaturated bonds (such as olefins) and aromatic rings of PAHs present in the residue oil. Using very high Mo concentrations (1800 ppm in the present study) significantly increases the number of hydrogenation sites, but the rate of hydrogen transfer to the liquid is limited. Hence the hydrogen uptake does not increase proportionally with the catalyst concentration at high catalyst concentrations. Figure 3 shows that unlike the coke yield, the H/C atom ratio in the products increased linearly with increased Mo concentration in the feed. Hydrogenation of PAHs reduced the amount of aromatics in the products, as shown in Figure 3, where the aromatic to aliphatic carbon ratio (CAr/CAl) in the product coke from experiments using the Mo-micelle precursor decreased rapidly with increased catalyst concentration. The H/ C atom ratios from two experiments, shown by the circle in Figure 3, were slightly offset from the line showing the correlation between the H/C atom ratio of the liquid product and the Mo concentration. This is because the H/C atom ratio of the liquid product depends on two factors, whereas the H/C atom ratio of the coke is mainly dependent on the hydrogen content of the macromolecular coke precursors which are more hydrogenated as catalyst concentration increases and hence produce more hydrogenated coke. In the liquid product on the other hand, the H/C atom ratio depends on both the extent of carbon removal (coke formation) and the degree of hydrogenation of the liquid phase. For the noncatalytic experiment, although the hydrogen uptake was low (Figure 2), a very high coke yield (Figure 1) resulted in a high H/C atom ratio of the liquid product. In the experiment with 100 ppm Mo using the Mo-micelle as the catalyst precursor, although the coke yield 2771
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was lower than the coke yield from the noncatalytic experiment (Figure 1), a higher hydrogen uptake increased the H/C atom ratio to the same extent as the noncatalytic experiment. Figure 4 reports the TIOR conversion and coke yield as a function of the Mo concentration used in the hydroconversion
Figure 4. Coke yield and residue (TIOR) conversion during CLVR hydroconversion as a function of Mo concentration for Mo-micelle and Mo-octoate catalyst precursors. Operating conditions: T = 445 °C, PH2 = 13.8 MPa at 900 mL(STP)/min, and 1 h reaction time.
of CLVR. The noncatalytic experiment had a much lower TIOR conversion compared to the catalytic experiments, due to a much lower hydrogen uptake in the absence of the catalysts (Figure 2). Low hydrogen uptake increases the coke formation significantly. When the Mo-micelle or Mo-octoate catalyst precursors were added to the reactor (even at very low concentrations of 100 ppm of Mo), the TIOR conversion increased significantly from 64 wt % in the noncatalytic experiment to 85−90 wt % in the catalytic experiments. The TIOR conversion remained almost constant as the catalyst concentration increased above 100 ppm Mo for both Mo precursors. This is because the residue conversion is a consequence of thermal cracking, followed by hydrogen transfer to the olefins produced by β-scission of the free radicals produced during cracking. These reactions occur in series starting with carbon−carbon bond cleavage followed by hydrogen transfer to unsaturated bonds and free radicals to prevent polymerization and condensation reactions. Since the cracking reaction is mainly controlled by temperature,62−65 the cracking reaction will be rate limiting during residue conversion and excess catalyst does not affect the extent of residue conversion. The HTSD data of Figure 5 reports the liquid product distribution according to standard BP ranges for naphtha (524 °C). The data show that higher catalyst concentration resulted in a small shift in the distribution of liquid products from naphtha toward light and heavy gas oils, both of which are more favorable products for refineries. Both the 100 ppm Mo and the noncatalytic experiment produced lighter liquid products with higher naphtha yield compared to experiments using higher catalyst concentrations. This shift in liquid product distribution was at
Figure 5. Liquid product distribution from CLVR hydroconversion as a function of Mo concentration for Mo-micelle (A) and Mo-octoate (B) catalyst precursors. Operating conditions: T = 445 °C, PH2 = 13.8 MPa at 900 mL(STP)/min, and 1 h reaction time.
the expense of a lower total liquid yield (including >524 °C fraction) and higher coke yield (Figure 1). The Mo-micelle and Mo-octoate precursors also showed very similar liquid product distributions, confirming that the catalyst precursors had no impact on the MoS2 catalyst activity. On the basis of the results reported herein, it may be concluded that increasing catalyst concentration above a certain value (600 ppm) does not have a significant impact on either the quality of the products or the coke suppression. To estimate the optimum concentration of catalyst in the semibatch reactor under the hydroconversion conditions used in the present study, the ratio of the TIOR conversion to the coke yield was evaluated. Maximizing this ratio reflects the ultimate goal of high residue conversion while coke production is minimized. Figure 6 shows the change in the TIOR conversion to coke yield ratio for different experiments of the present study using both Mo-micelle and Mo-octoate precursors. Figure 6 shows that the ratio increased significantly when the catalyst was added to the reactor, up to 600 ppm Mo concentration in the reactor. At 1800 ppm Mo concentration, the ratio decreased marginally. Although the two precursors showed very similar activity in terms of hydroconversion of the residue feed, the Mo-octoate precursor would be preferred. This is due to the simplicity of using an oil-soluble precursor compared to the Mo-micelle 2772
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conversion experiments in which a 1-h reaction time followed the heat-up period, each done at different reaction temperatures. Comparison of experiments B and D with experiments C and E clearly shows that at the higher temperature, the heatup period had a much more significant effect on the coke yield, asphaltene and TIOR conversions, and liquid-phase sulfur conversion, compared to the experiments done at lower temperatures. Heat-up to 445 °C resulted in 62% residue conversion and 1.9 wt % coke yield. The data of Table 1 show that at 430 °C, 43% of the total coke produced was formed during the heat-up period and this increased to 65% at 445 °C (comparing experiments B with C and D with E). Large aliphatic fragments attached to the polyaromatic core in residue macromolecules have a tendency to crack very rapidly.63 Due to the very high rate of cracking when the concentration of these fragments is high (during the heat-up period), the rate of hydrogen transfer to the cracked fragments is slower than the rate of condensation to coke particles. Comparing the data of experiments A, C, and E in Table 1 also shows that increasing the reaction temperature from 415 to 430 °C and then to 445 °C increased the coke yield and the TIOR, asphaltene, and S conversions. 3.3. Coke Properties. The coke recovered from the experiments of the present study was characterized to investigate the effect of catalyst on the coke properties. Since after reaction most of the catalyst added to the reactor was associated with the product coke,58 understanding the produced coke morphology and the coke−catalyst interaction is necessary if the catalyst is to be recycled in slurry phase hydroconversion. The BET surface area of the coke generated at different operating conditions is presented in Table 2 and compared to
Figure 6. TIOR conversion to coke yield ratio from CLVR hydroconversion as a function of Mo concentration for Mo-micelle and Mo-octoate catalyst precursors. Operating conditions: T = 445 °C, PH2 = 13.8 MPa at 900 mL(STP)/min, and 1 h reaction time.
precursor which requires several synthesis steps. In addition, the lower cost of the commercially available Mo-octoate precursor, compared to the Mo-micelle that requires the use of expensive surfactants, would be a significant advantage. 3.2. Effect of Temperature and Heat-up Period. Heck et al.63 showed that part of the residue fraction in residue oil has a tendency to crack very rapidly. Since hydroconversion reactions occur at temperatures above 350 °C4 and the standard reaction temperature of the present study was 445 °C, the hydroconversion reaction must have also occurred during heat-up of the reactor. For the semibatch slurry-phase reactor used in the present study, the heat-up period from 350 to 445 °C was approximately 20 min. Consequently, it was important to determine the effect of the heat-up period and reaction temperature on the residue oil hydroconversion. Since the Momicelle and Mo-octoate precursors showed very similar activity, these experiments were done using the Mo-micelle precursor. Table 1 summarizes the reaction times and reaction temperatures investigated to determine the effect of the heatup period. In experiments B and D, following heat-up of the reactor to 430 and 445 °C, respectively, the reactor was quenched immediately and the products recovered and analyzed. Experiments A, C, and E were standard hydro-
Table 2. BET Surface Area and Total Pore Volume of Coke Recovered from Noncatalytic and Catalytic CLVR Hydroconversion Experimentsa experiment for coke source sample BET surface area, m2/g BJH total pore volume, cm3/g
experiment identifier
A
B
C
D
E
415 1 0.68 52.09 43.33 47.67
430 0 1.08 47.90 38.32 35.00
430 1 2.52 69.82 66.20 59.33
445 0 1.88 61.80 50.01 43.33
445 1 2.90 83.74 75.13 67.29
noncatalytic
3.9
4.9
0.018
0.028
600 ppm Mo-micelle 13.5 0.063
600 ppm Mo-octoate 18.5 0.075
Reaction conditions: T = 445 °C, PH2 = 13.8 MPa at 900 mL(STP)/ min, and 1 h reaction time.
a
the BET surface area of pure graphite (Aldrich, mesh 325, 99.999 wt %). The coke recovered from a noncatalytic experiment had a low BET surface area (4.9 m2/g), comparable to that of graphite (3.9 m2/g), whereas the BET area of the coke generated by catalytic hydroconversion was significantly higher. The coke recovered from the two hydroconversion experiments using 600 ppm Mo-micelle and Mo-octoate precursors had BET areas of 13.5 and 18.5 m2/g, respectively. The BJH total pore volume of coke samples had similar trends. The results indicate that the coke was more amorphous and porous in the presence of the MoS2 catalyst, compared to the coke recovered from the noncatalytic experiment. This is probably due to the higher hydrogen content of the coke generated during catalytic hydroconversion (Figure 3) which reduced the crystallinity of the carbonaceous material and increased the surface area. These observations are consistent
Table 1. Coke Yield, TIOR, and Asphaltene Conversion and Liquid-Phase Sulfur Conversion in the Heat-up Experiments and the Experiments using Different Reaction Temperaturesa
reaction temperature, °C reaction time,b h coke yield, wt % TIOR conversion, wt % asphaltene conversion, wt % sulfur conversion, wt %
commercial graphite
a Experimental conditions: PH2 = 13.8 MPa at 900 mL(STP)/min. Catalyst used for all experiments was 600 ppm Mo-micelle. bReaction time after heat-up to the desired reaction temperature.
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with XRD analysis of the coke reported previously58 that showed an intense graphite peak for coke recovered from a noncatalytic experiment that became less intense and broader as catalyst concentration increased. The type of catalyst precursor seemed not to have a significant effect on the crystalline structure of the carbon and the MoS2 in the coke−catalyst mixture recovered after reaction. Figure 7 shows the XRD diffractogram of the coke recovered
Figure 8. TEM micrographs of coke samples recovered from CLVR hydroconversion using different concentrations of Mo-octoate as catalyst. (A) Noncatalytic experiment, (B) 600 ppm Mo-octoate as catalyst, and (C) 1800 ppm Mo-octoate as catalyst. Operating conditions: T = 445 °C, PH2 = 13.8 MPa at 900 mL(STP)/min, and 1 h reaction time.
Figure 7. XRD diffractogram of coke recovered from CLVR hydroconversion using 1800 ppm Mo from Mo-micelle and Mooctoate catalyst precursors. Operating conditions: T = 445 °C, PH2 = 13.8 MPa at 900 mL(STP)/min, and 1 h reaction time.
DRIFTS spectra of the coke−catalyst are presented in Figure 9. Three main frequency regions can be distinguished in which bands occur (2800−3000, 1300−1500, and 700−1300 cm−1) together with an intense and relatively broad band at 1600 cm−1. These bands are in agreement with literature on coke deposition on catalysts.67−73 In the range of the C−H stretching vibration in alkanes (region of 2800−3000 cm−1 or
from two hydroconversion experiments with 1800 ppm of Mo added as Mo-micelle and Mo-octoate precursors. Iron and nickel sulfides were observed as very sharp peaks with similar intensities for both samples. Due to possible washout of metal particles from the reactor wall during coke recovery, comparison of the iron and nickel sulfide peaks between different coke samples is not meaningful. A noticeable amount of ammonium chloride (NH4Cl) was detected when the Momicelle precursor was used. Figure 7 shows a relatively broad peak of MoS2 at ∼59° and the absence of a sharp peak from the (002) plane of MoS2 at ∼14°, indicative of a high dispersion of MoS2 with a very small number of crystalline layers.66 Such a structure was confirmed by TEM analysis (Figure 8). In Figures 8B and C, single layers of MoS2 can be observed within the coke samples. The number of these single layers is clearly higher in Figure 8C, corresponding to the coke recovered when using 1800 ppm Mo prepared from the Mo-octoate precursor compared to the 600 ppm Mo of Figure 8B. Although the concentration of MoS2 in the coke recovered from an experiment using 1800 ppm Mo using the Mo-octoate precursor was higher than the MoS2 concentration in the coke recovered from the experiment using 600 ppm Mooctoate precursor, the MoS2 catalyst remained single layered and highly dispersed within the coke. Furthermore there was no sign of catalyst agglomeration or stacking of the MoS2 layers with increased catalyst concentration. Figure 8 also shows the difference in morphology of the coke recovered from noncatalytic and catalytic experiments. The TEM micrographs of the coke recovered from the noncatalytic experiments clearly showed the presence of graphitic structures (Figure 8A, where a graphitic particle of diameter ∼40 nm is clearly visible) whereas no such features were present in the coke samples recovered from the catalytic experiments.
Figure 9. DRIFT spectra of coke samples recovered from CLVR hydroconversion using different concentrations of Mo added in the form of Mo-octoate. Operating conditions: T = 445 °C, PH2 = 13.8 MPa at 900 mL(STP)/min, and 1 h reaction time. Reaction time for heat-up experiment was zero and Mo concentration was 600 ppm using Mo-octoate precursor. 2774
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Table 3. TGA of Coke Recovered from Hydroconversion Reactions Using Different Types of Catalysts at Different Concentrations Mo-micelle precursor
Mo-octoate precursor weight loss,a %
a
Mo, ppm
120−550 °C
550−750 °C
750−900 °C
120−900 °C
120−550 °C
550−750 °C
750−900 °C
120−900 °C
0 100 300 600
4.31 N/A 10.50 16.03
5.20 N/A 5.62 5.36
3.36 N/A 7.38 9.44
12.87 N/A 26.89 30.83
4.31 9.85 11.35 N/A
5.20 6.80 6.23 N/A
3.36 6.76 6.86 N/A
12.87 23.41 24.44 N/A
Weight loss = [(Wi − Wf )/Wi ] × 100[%]. Wi: weight of the coke at initial temperature. Wf: weight of the coke at final temperature.
spectrum of the coke recovered from the heat-up experiment did not show any distinguishable peaks at these wave numbers. Although the DRIFTS spectra showed a clear change in the region 1000−1300 cm−1 (peak intensity increased as the catalyst concentration increased), no conclusion could be made regarding these bands mainly because the peaks were not well resolved. TGA analysis of the coke recovered from the noncatalytic and catalytic experiments using Mo concentrations 100, 300, and 600 ppm are presented in Table 3. The coke was recovered from experiments in which both Mo-micelle and Mo-octoate precursors were used as the catalyst precursor. The results clearly show differences between the coke recovered from noncatalytic and catalytic experiments. Using both Mo-octoate and Mo-micelle precursors, the coke loses much more weight compared to the coke recovered from the noncatalytic experiment. Due to the high temperature reached during the TGA analysis in N2, cracking reactions occur and carryover of the cracked products causes the weight loss. Hence the results indicate that the coke recovered from the catalytic experiments undergo cracking reactions much more readily than the coke from noncatalytic experiments, probably due to the more graphitic nature of the coke recovered from the noncatalytic experiments. The weight loss increased marginally as the catalyst concentration increased, with the total weight loss of 26.9% with 300 ppm Mo increasing to 30.8% with 600 ppm Mo. XPS and EDX characterization data of the coke is summarized in Table 4. EDX analysis showed that the Mo concentration in the coke increased from 1.28 to 2.82 wt % as the catalyst concentration increased from 300 to 600 ppm using the Mo-micelle catalyst precursor. The Mo/C weight ratio in the coke was 0.02 for the coke recovered from the experiment using 300 ppm Mo and this ratio doubled as the catalyst concentration doubled. Although the same Mo concentration (600 ppm) was used in both heat-up experiments B and D, the Mo/C weight ratio increased to 0.09 and 0.06 for the heat-up experiment to 430 and 445 °C, respectively. This could be because the coke samples recovered from the heat-up experiments had higher H/C atom ratio (Figure 10) than the coke samples recovered after 1 h reaction. More likely, however, is the fact that less coke was generated in the heatup experiments compared to after 1 h of reaction, yielding a higher Mo/C weight ratio. The decreased Mo/C weight ratio as the reaction temperature increased from 430 to 445 °C is also consistent with more coke generation and a lower H/C ratio of the recovered coke at higher temperatures because of more cracking and removal of lighter and more hydrogenated parts of the residue.
paraffinic region), three main bands were observed at 2862, 2917, and 2962 cm−1. These are assigned to the symmetric vibrations of CH2 and CH3, the asymmetric vibration of CH2, and the asymmetric vibration of CH3, respectively.72 The bands are distinguishable in all spectra of Figure 9. In the coke samples recovered after 1-h reaction with catalyst concentrations of 100, 300, and 1800 ppm Mo using the Mo-octoate precursor, the ratio of the band intensities at 2917 and 2962 cm−1 was almost constant, implying that the length of the aliphatic branches in the coke samples were similar. The coke recovered from the noncatalytic experiment had a lower ratio indicative of shorter aliphatic chains. On the other hand, in the coke recovered after heat-up to 445 °C (top spectrum in Figure 9), the band at 2917 cm−1 was of much higher intensity compared to the band intensity at 2962 cm−1. Clearly after 1 h of reaction, a portion of the aliphatic chains of the residue oil are cracked and separated from the macromolecules to produce gas or liquid, depending on the length of the chain. This makes the aliphatic chain length of the coke precursors shorter after 1 h of reaction compared to after the heat-up period. The band at 1600 cm−1 in the DRIFTS spectra is attributed to the stretching vibration of CC in microcrystalline graphitic structures, which are present in polycyclic aromatic compounds.69,72,74 The 3050 cm−1 band is probably due to the C H stretching vibration in aromatics.75,76 Although the band at 1600 cm−1 had almost the same intensity in all the coke samples, there was a decreasing trend in the intensity of the aromatic band at 3050 cm−1 with increasing Mo concentration. Hence it may be concluded that with a decrease in catalyst concentration the concentration of aromatic rings in the recovered coke increased. In the range of the CH bending vibration (region of 1300−1500 cm−1) in paraffinic or olefinic compounds, the band at 1442 cm−1 can be assigned to the asymmetric bending vibration of aliphatic groups attached to aromatic rings. This band usually replaces the in-plane bending vibration of CH2 at ∼1465 cm−1 when aromatics are formed.72,76 The band at 1378 cm−1 originated from the symmetric bending vibration of CH3 and the band at 1325 cm−1 is an unknown absorption. These bands are distinguishable in the DRIFTS spectra of the coke, and Figure 9 shows that they are almost identical for all the coke samples analyzed. In the region of 7001000 cm−1, three main absorption bands are distinguished at 751, 811, and 866 cm−1. These likely originate from the in-phase out-of-plane wagging vibrations of hydrogen in substituted aromatic rings.75,76 The peaks were very intense in the coke recovered from the noncatalytic experiment. As the catalyst concentration increased to 1800 ppm Mo, using the Mo-octoate precursor, the intensity of the peaks decreased and finally disappeared. The DRIFTS 2775
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during further reaction. The high coverage of MoS2 by solid coke suggests that the MoS2 may act as a nucleation site for coke formation. 3.4. Catalyst Recycle. Although unsupported MoS 2 catalysts have shown very high activity in terms of coke suppression and residue conversion, using these catalysts once through in the upgrading process results in high catalyst costs. One approach to decreasing these costs is to recycle the catalyst. In the present study, the solid coke−catalyst recovered from an experiment in which 600 ppm Mo, using the Momicelle precursor, was selected for recycle. Since previous work demonstrated that all the Mo added to the residue feed was present in the recovered coke,58 a Mo concentration of 600 ppm in the residue feed would result if all the recovered coke was added to the reactor in a subsequent hydroconversion experiment to simulate catalyst recycle. In the present study, the recovered coke was divided into 3 portions and mixed with Mo-micelle to yield a fixed total Mo concentration of 300 ppm, made up of 0, 33, 67, and 100% recycled Mo. Results of residue oil hydroconversion using the different portions of recycled catalyst are compared in Table 5. The data
Table 4. EDX and XPS Analysis of Coke Generated from CLVR Hydroconversion Experiments in the Semibatch Reactor EDX analysisa coke sample 300 ppm MoMd 600 ppm MoM 600 ppm MoOe heat-up Bc heat-up Dc
XPS analysis
coke yield, wt %
Mo, wt %b
Mo, wt %
Mo/C
Mo, wt %
Mo/C
1.91
1.57
1.28
0.02
N/A
N/A
1.83
3.27
2.82
0.04
0.22
0.002
1.76
3.41
3.22
0.04
0.25
0.002
1.08 1.88
5.56 3.19
4.70 4.92
0.09 0.06
0.54 0.46
0.007 0.005
a
EDX analysis data presented are average of at least six different data points. bTheoretical weight percent of Mo in the coke assuming all the coke added to the reactor will end up in the recovered solid coke. c Heat-up experiment letters are based on letters in Table 1. In both heat-up experiments, 600 ppm Mo-micelle was used as catalyst precursor. dMoM: Mo-micelle used as catalyst precursor. eMoO: Mooctoate used as catalyst precursor.
Table 5. Results from CLVR Hydroconversion Using Different Ratios of Fresh and Recycled Catalysta Mo concentration, ppm fresh recycle H2 uptake, wt % gas yield, wt % coke yield, wt % H2S yield, wt % liquid H/C atom ratio liquid yield, wt % naphtha light gas oil heavy gas oil residue TIOR conversion
Figure 10. H/C atom ratio comparison of coke samples recovered from heat-up and 1 h reaction time CLVR hydroconversion at different reaction temperatures using 600 ppm Mo-micelle. Experimental conditions are defined in Table 1.
exp 1
exp 2
exp 3
exp 4
300 0 2.21 5.83 1.91 1.12 1.53 88.03 21.41 32.10 24.16 10.36 83.60
200 100 2.27 6.01 1.77 1.11 1.53 87.05 20.84 32.33 24.23 9.65 84.74
100 200 2.19 5.87 1.64 1.04 1.53 87.71 21.32 32.20 24.40 9.79 84.73
0 300 2.42 6.08 2.22 1.43 1.53 83.77 23.47 30.69 21.31 8.29 85.55
Reaction conditions: T = 445 °C, PH2 = 13.8 MPa at 900 mL(STP)/ min, and 1 h reaction time.
a
show that the recycled catalyst had very similar activity to that of the fresh Mo precursor in terms of coke suppression, hydrogen uptake, gas yield, and H2S yield. Increasing the fraction of recycled catalyst in the total catalyst added to the reactor did not have any significant effect on the catalyst activity. The coke yield from all four experiments was
