Effect of Catalyst Deactivation and Reaction Time on Hydrocracking

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Energy & Fuels 2007, 21, 1370-1378

Effect of Catalyst Deactivation and Reaction Time on Hydrocracking Heavy Hydrocarbon Liquids Marcos Millan,* Cristina Adell, Cecilia Hinojosa, Alan A. Herod, Denis Dugwell, and Rafael Kandiyoti Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK ReceiVed September 15, 2006. ReVised Manuscript ReceiVed January 25, 2007

The activity and fouling behavior of a commercial NiMo/Al2O3 catalyst has been studied during the hydrocracking of a coal extract at short reaction times. The catalyst was precoated with a carbonaceous deposit beforehand in order to study the interaction between a coated catalyst and fresh feed. The conversion of the heavier fraction (boiling point above 450 °C) of the feed steadily increased with time. However, the progression in the amount of carbonaceous deposits on the coated catalyst was not uniform. A large initial deposition was observed when the catalyst and fresh feed were placed in contact. This initial deposition was reversible, and within the first 10 min of reaction, much of the deposit redissolved into the liquid phase with increasing reaction times. This bulk exchange of material between the deposits and the solution appears to be a mechanism which would help explain the sustained level of catalytic activity despite the large carbonaceous deposition. A “harder” more permanent deposit layer built up more gradually as reaction time increased.

1. Introduction Catalysts employing molybdenum promoted by either cobalt or nickel, supported on alumina, are commonly employed for hydroprocessing in the oil industry. These catalysts suffer certain degrees of deactivation during the first hours on stream and settle down to operate at lower levels of activity. Their availability and widespread use make them suitable reference catalysts in the study of more novel catalytic hydrocracking methods. Catalyst deactivation has a direct impact on the economic viability of heavy hydrocarbon upgrading processes, such as coal liquefaction and heavy oil hydroprocessing. As the necessity for heavy hydrocarbon processing grows, the necessity for understanding the processes involved in catalyst deactivation becomes more compelling. Fouling, through the deposition of heavy hydrocarbons on active surfaces and pore plugging, is thought to be responsible for the quick loss of catalytic activity affecting both the hydrogenation and cracking functionalities of NiMo based catalysts.1 The rate and extent of the deactivation processes appear to be strongly dependent on the characteristics of the sample being processed; deactivation is usually slower during the hydroprocessing of light feeds. A commercial NiMo/Al2O3 commonly used in the oil industry as a hydrotreating catalyst has been extensively tested in this laboratory, during hydrocracking studies of several heavy petroleum-derived liquids and coal liquefaction extracts.2-5 Zhang et al.2,6 investigated the extent of hydrocracking of a coal liquefaction extract as a function of reaction time. * Corresponding author. E-mail address: [email protected]. (1) Furimsky, E.; Massoth, F. E. Deactivation of hydroprocessing catalysts. Catal. Today 1999, 52, 381-495. (2) Zhang, S.-f.; Xu, B.; Herod, A. A.; Kandiyoti, R. Hydrocracking reactivities of primary coal extracts prepared in a flowing-solvent reactor. Energy Fuels 1996, 10, 733-742. (3) Zhang, S.-f.; Herod, A. A.; Kandiyoti, R. Effectiveness of dispersed catalysts in hydrocracking a coal liquefaction extract: a screening study. Fuel 1997, 76, 39-49.

During initial experiments, high conversions were observed for fractions of the feed with a boiling range above 450 °C (the “>450 °C” fraction), during the first 30 min of hydrocracking. This stage was followed by a continuous slowdown in the reaction rate up to 120 min.2,6 No significant changes in conversion were detected at reaction times longer than 120 min. In principle, this decrease in conversion rate could have been due to catalyst deactivation. However, an alternative explanation would be that in the batch system used in those studies, the most readily hydrocrackable fractions of the sample reacted within the first 120 min, leaving only the most intractable materials. This would lead to loss of sample reactivity of the batch, which would be undistinguishable from catalyst deactivation. Both factors were evaluated in subsequent work by Begon et al.,7,8 and the loss of sample reactivity was identified as the predominant cause of the observed slowdown in reaction rates. Although some degree of catalyst deactivation could be directly observed, the catalyst was still considerably active after 6 h of reaction. The changes in the extent of hydrocracking during the early stages of the reaction (times shorter than 30 min) have also been addressed by Begon et al.7 A somewhat unexpected pattern of hydrocracking conversions was observed within the first 10 min of the reaction. A drop in the >450 °C boiling fraction (4) Bodman, S. D.; Mc Whinnie, W. R.; Begon, V.; Suelves, I.; Lazaro, M. J.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Metal-ion pillared clays as hydrocracking catalysts (I): Catalyst preparation and assessment of performance at short contact times. Fuel 2002, 81, 449-459. (5) Bodman, S. D.; Mc Whinnie, W. R.; Begon, V.; Millan, M.; Suelves, I.; Lazaro, M. J.; Herod, A. A.; Kandiyoti, R. Metal-ion pillared clays as hydrocracking catalysts (II): effect of contact time on products from coal extracts and petroleum distillation residues. Fuel 2003, 82, 2309-2321. (6) Zhang, S.-f.; Xu, B.; Moore, S. A.; Herod, A. A.; Kandiyoti, R. Comparison of hydrocracking reactivities of coal extracts from a flowingsolvent reactor, a mini-bomb and a pilot plant. Fuel 1996, 75, 597-605. (7) Begon, V.; Megaritis, A.; Lazaro, M. J.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Changes in sample reactivity and catalyst deactivation during early stages of the hydrocracking of a coal extract. Fuel 1998, 77, 1261-1272. (8) Begon, V. Ph.D. thesis, Imperial College, University of London, 1998.

10.1021/ef060466o CCC: $37.00 © 2007 American Chemical Society Published on Web 03/17/2007

Hydrocracking HeaVy Hydrocarbon Liquids

content took place during the heatup of the sample to 440 °C. Conversion increased in the next 5 min to show a maximum, which was followed by a diminution leading to a minimum in conversion at 10 min. After this period, the conversion was observed to increase monotonically as described in earlier reports.2,6 The minimum in conversion observed at 10 min was thought to be associated with the repolymerization of reactive molecules and radicals released through thermal cracking during heatup and the first 5 min of reaction time. It would appear that, during this early stage, the activation (by sulfidation) of the fresh NiMo/ Al2O3 catalyst was still in progress and the catalyst not yet fully active.7 Deposition on the catalyst was found to be strong when fresh catalyst was placed in contact with fresh feed.7 The carbonaceous layer coating the catalyst after a “heatup only” run (0 min of holding at the peak temperature) represented almost 20% of the total filter cake weight. The amount of deposits steadily decreased with reaction time, and after the 120 min run, only 11% of the filter cake consisted of carbonaceous material. This suggested a process of reaction and redissolution of the deposits in the liquid phase. When an already coated catalyst was reused in contact with fresh feed in successive 120 min experiments, some new deposition took place, leading to an increase in the total amount of carbonaceous deposit on the catalyst. However, the amount of deposit added to the carbonaceous layer became progressively smaller; thermogravimetric analysis (TGA) combustion tests on the solid deposits showed that the carbonaceous layer tended to become less reactive. The process was slow and in apparent agreement with observations suggesting the relative insensitivity of the amount of deposits to time on stream in continuous reactors.9-11 The early stages of the hydrocracking process are addressed again in the present work with two key modifications to experimental design. First, the Point of Ayr coal extract (already containing recycle solvent) is hydrocracked in the absence of tetralin as solvent. This allowed the examination of products free of tetralin and products of the reactions of tetralin. Reaction conditions were therefore closer to conditions in the larger (pilot) scale process, where no extra hydrogen donor compound was added to the coal extract. Second, already coated NiMo/Al2O3 catalyst samples have been used to study the processes taking place in the first few minutes of contact between a coated catalyst and the fresh feed. In previous experiments, rapid early deposition on fresh catalyst surfaces tended to skew product compositions. Precoating catalyst samples thus serves to better represent (in a batch reactor) the process where fresh feed is continuously added to the pilot reactor and comes in contact with a catalyst that has already seen a level of fouling and deactivation. 2. Experimental 2.1. Hydrocracking Experiments. The microbomb reactor and the experimental procedure have been described elsewhere.2,6 This reactor is operated in batch mode. A 1 g portion of hydrocracker feed from the liquid solvent extraction process development pilot (9) Richardson, S. M.; Nagaishi, H.; Gray, M. R. Initial Coke Deposition on a NiMo/γ-Al2O3 Bitumen Hydroprocessing Catalyst. Ind. Eng. Chem. Res. 1996, 35, 3940-3950. (10) Benito, A. M.; Martinez, M. T. Catalytic hydrocracking of an asphaltenic coal residue. Energy Fuels 1996, 10, 1235-1240. (11) Matsushita, K.; Hauser, A.; Marafi, A.; Koide, R.; Stanislaus, A. Initial coke deposition on hydrotreating catalysts. Part 1. Changes in coke properties as a function of time on stream. Fuel 2004, 83, 1031-1038.

Energy & Fuels, Vol. 21, No. 3, 2007 1371 plant at Point of Ayr12 and 250 mg of a previously coated NiMo/ Al2O3 catalyst (see below) were charged into a 5 mL reactor. No solvent was added to the reaction mixture. Reaction time, defined as the holding time at 440 °C, was varied from 0 (heatup immediately followed by cooldown) to 120 min. Independent experiments were run for each reaction time. This was done in order to avoid taking out sample during the experiment, which could affect the results due to the relatively small amount of sample used. 2.2. Precoating of Catalyst with Carbon Deposit. The NiMo/ Al2O3 catalyst was coated with carbonaceous materials beforehand. The starting material was fresh PBC-90D. This catalyst consists of an active phase of MoO3 (8 wt %) and NiO (4 wt %) dispersed on a γ-Al2O3 support. It is preimpregnated with a proprietary organic compound containing sulfur, whose composition is kept confidential by the catalyst supplier. This compound releases H2S during heatup enabling catalyst activation. The particle size of this commercial catalyst was less than 250 µm. A 1.5 g portion of coated NiMo/Al2O3 was prepared in two steps. First, 900 mg of the catalyst and 1.5 g of a coal tar pitch were charged into the microbomb reactor and kept during 2 h at 350 °C and 170 bar hydrogen pressure. That process was repeated to duplicate the amount of coated catalyst available. The solids recovered from the two runs were washed with N-methyl-2pyrrolidinone (NMP) to remove soluble materials and mixed. In a second step, 1 g of the coal tar pitch was mixed with 1.5 g of the catalyst mixture recovered from the previous duplicated runs. The reaction took place at 370 °C and 170 bar hydrogen pressure. NMP was again used to wash the reaction mixture. The process was repeated to generate larger amounts of coated catalyst. The recovered catalyst was heavily coated; its carbonaceous content was between 30 and 35% of the total solids. 2.3. Thermogravimetric Analysis. A Perkin-Elmer TGA7 thermogravimetric analyzer was used to determine both the >450 °C fraction of the liquid products, (f>450C)liq, and the feed, (f>450C)feed, and the carbonaceous deposition on the catalyst, fcarb. These values are applied in the calculation of conversion as shown below. 2.3.1. ThermograVimetric Analysis: >450 °C Fraction. Conversions of the >450 °C fraction were obtained from TGA data by a method described in detail elsewhere.2 Briefly, it consists of heating the sample under a flow of helium and measuring the fraction of the sample evaporated as a function of temperature on the TGA pan. A calibration developed by Zhang et al.13 is applied to relate the evaporation temperature on the TGA pan with the normal boiling point. According to this calibration, a TGA temperature of 247 °C corresponds to a boiling point of 450 °C. However, the use of no solvent in these hydroracking experiments allowed modifications to the temperature program to be introduced. The original temperature program included a 2 h step in which the sample was kept at 50 °C in order to evaporate tetralin. This step has been eliminated to adapt the method to runs without tetralin. Instead, the sample is heated directly from 30 to 247 °C, shortening the analysis time. In addition, as no tetralin is introduced in the system, TGA data obtained in the region of boiling points below 450 °C is free from solvent-derived materials. This enabled the generation of boiling point distributions in this region. They are obtained as the derivative of the TGA weight vs temperature curve. 2.3.2. ThermograVimetric Analysis: Carbonaceous Deposition on the Catalyst. The carbonaceous deposits are burnt in a TGA under a flow of air. Their fraction in the recovered catalyst is calculated by difference with the remaining weight on the TGA pan. The sample is heated up to 600 °C and kept at that temperature for 30 min. Details on the procedure have been presented elsewhere.14 (12) Kimber, G. M. Energy for the future - coal liquefaction for the European EnVironment; Report No. Coal R078, Department of Trade and Industry: United Kingdom, 1997. (13) Zhang, S.-f. Ph.D. thesis, Imperial College, University of London, 1995.

1372 Energy & Fuels, Vol. 21, No. 3, 2007 As an already coated catalyst is reused in this work, the amount of material added to the carbonaceous layer needs to be calculated. From TGA measurements, the carbonaceous contents as a fraction F of the total solids before ( f °carb and after ( f carb ) the run are known. The fraction of carbonaceous layer due to new deposition on the catalyst “∆fcarb” can be calculated as follows: F F ∆fcarb ) f carb - f °carb(1 - f carb )/(1 - f °carb)

This value of ∆fcarb is added to the >450 °C fraction in the liquid products in runs where the catalyst has been reused. 2.4. Definition of Apparent and Real Conversions. The “apparent conversion” is obtained by taking into account only the >450 °C fraction in the liquid products (CHCl3/CH3OH soluble): Apparent Conversion )

mfeed(f>450C)feed - mliq(f>450C)liq mfeed(f>450C)feed

where mfeed and mliq are the weight of the feed and the liquid recovered after hydrocracking, respectively. The terms (f>450C)feed and (f>450C)liq are the fractions of material with boiling points above 450 °C in the feed and liquid products, respectively. As high apparent conversions could be achieved by removal of >450 °C material as deposits on the catalyst just as well as by hydrocracking, the evaluation of the catalyst effectiveness cannot only be based on this value. The solids deposited on the catalyst are considered as part of the >450 °C fraction of the products to calculate the real conversion. It is defined as follows: Real Conversion )

mfeed(f>450C)feed - mliq(f >450C)liq - msol∆fcarb mfeed(f>450C)feed

where msol is the weight of solids recovered and ∆fcarb is the carbonaceous fraction of the solids deposited during the run. 2.5. Size Exclusion Chromatography (SEC). The SEC system has been described elsewhere.15 Briefly, a polystyrene/polydivinylbenzene SEC column from Polymer Laboratories Ltd (Mixed-A column) was used in this work. The porosity range of this column is not available due to manufacturer confidentiality. The linear range of log MM vs retention time is up to 15 million for this column, according to a calibration based on polystyrene standards. The calibration of SEC columns has been discussed in detail elsewhere.16 A 0.5 mL/min flow of NMP is pumped through the system, which remains at room temperature. The signal is detected by ultraviolet absorption at five wavelengths 280, 300, 350, 370, and 450 nm. The latter is recorded by a variable wavelength PerkinElmer LC250 UV detector set at 450 nm whereas the rest are measured by a diode array detector. An evaporative light scattering detector is also connected in series with the UV ones. Data from all six channels are collected, displayed, and saved in a PC. Software developed in this laboratory is applied to normalize intensities in order to allow comparison among runs where different amounts of material were injected. In this work, area normalization was applied to chromatograms recorded by the 350 nm UV-A detector. 2.6. UV Fluorescence Spectroscopy. A Perkin-Elmer LS50 luminescence spectrometer has been used in the static-cell mode. The UV-F spectra have been recorded in three modes: synchronous, emission, and excitation. The emission spectrum is obtained by (14) Begon, V.; Warrington, S. B.; Megaritis, A.; Charsley, E. L.; Kandiyoti, R. Composition of the carbonaceous deposits and catalyst deactivation in the early stages of the hydrocracking of a coal extract. Fuel 1999, 78, 681-688. (15) Lazaro, M. J.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Calibration of Size Exclusion Chromatography in 1-Methyl-2-Pyrrolidinone for CoalDerived Materials Using Standards and Mass Spectrometry. Energy Fuels 1999, 13, 1212-1222. (16) Karaca, F.; Islas, C. A.; Millan, M.; Behrouzi, M.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. The calibration of size exclusion chromatography columns: Molecular mass distributions of heavy hydrocarbon liquids. Energy Fuels 2004, 18, 778-788.

Millan et al. exciting the sample at a constant wavelength and recording the emission in the full range of wavelengths. The excitation spectrum is recorded at a fixed wavelength while the excitation frequency is changed over the range. In the synchronous spectrum, both excitation and emission are simultaneously varied but the wavelength difference is kept constant at 20 nm. In this mode, the fluorescence of sample solutions in NMP has been measured between 250 and 800 nm. The samples were scanned at 240 nm/ min. 2.7. Specific Surface Area Measurements. A Micromeritics 2000 ASAP surface area analyzer was used for specific surface area measurements. Approximately 200 mg of sample were employed in each analysis. The standard method based on the adsorption of nitrogen on the surface was applied.17 The samples were dried overnight at 150 °C in a vacuum oven prior to the experiment to remove moisture and adsorbed gases from the catalyst pores. The model developed by Brunauer, Emmett, and Teller (BET model) was applied to calculate the surface area. Pore size distributions were obtained by the BJH method (developed by Barret, Joyner, and Halenda) based on the desorption isotherm. Specific surface area measurements were applied to the fresh NiMo/Al2O3, the coated NiMo/Al2O3 used as catalyst in these runs, and the catalysts recovered from the 0, 10, and 30 min runs.

3. Results 3.1. Boiling Point Distributions. As already mentioned, experiments in the absence of a donor-solvent (e.g., tetralin) in the feed to the reactor allow solvent-free data to be collected in the full range of boiling point distributions. These distributions have been obtained from the derivatives of the TGA weight vs temperature curves used to determine the boiling point distribution of the feed and the products. The data exhibited a marked dependence on the weight loaded on the TGA pan. Larger amounts of sample on the TGA pan delayed the evaporation of the sample, which translated into a shift in the distributions toward higher boiling points. It has been observed that a difference of 2 mg in weight loaded could produce shifts in the boiling point distributions of around 25 °C. Although in the present experimental array the amount of sample loaded in the TGA pan cannot be controlled with sufficient precision, the method can still be useful in indicating general trends between samples. This is exemplified in Figure 1 where boiling point distributions of the feed and the 0 (heatup only) and 10 min hydrocracked products are presented. The feed showed a larger proportion of material in the relatively high boiling point end of the distribution (peak with a maximum around 350 °C) than the 0 and 10 min hydrocracked products. The boiling point distribution has shifted toward lower values from feed to products, which show a larger proportion of their material under the peak at around 250 °C. 3.2. Conversions. Table 1 shows the apparent and real conversions of the >450 °C boiling fraction as a function of reaction time. As described above, the difference between these two conversions resides in the fact that the “apparent” conversion does not consider the carbonaceous deposits as part of the >450 °C boiling fraction of the products. By contrast, deposits are taken into account as part of the >450 °C boiling materials in the calculation of the “real” conversion. Hydrocracking of this fraction exhibited some progress during the reactor heatup to 440 °C. Here, 13% of the >450 °C boiling fraction was converted into lower boiling point materials within (17) McDonnell, M. E.; Walsh, E. K. In A guide to materials characterization and chemical analysis; Sibila, J. P., Ed.; VCH Publishers: New York, 1988; pp 257-261.


Energy & Fuels, Vol. 21, No. 3, 2007 1373

Figure 1. Boiling point distribution of the Point of Ayr hydrocracking feed and the 0 and 10 min hydrocracked products obtained by differential thermogravimetry (DTG). The distributions have been area normalized. Table 1. Apparent and Real Conversions of the 450 °C Fraction and Deposition on the Catalyst as a Function of Reaction Timea time (min)

apparent conversionb (%)

new carbonaceous deposits (% of filter cake)

real conversionc (%)

0 5 10 30 60 120

26 25 16 26 29 45

18 14 0 6 -2 12

13 14 16 22 30 37

a Experiments were carried out at 440 °C and 190 bar hydrogen pressure. A previously coated catalyst was employed in these experiments. b Percent change of original >450 °C boiling material in the liquid phase. c Percent change of original >450 °C boiling material in the liquid phase minus fresh carbon caught up on the catalyst.

that period. The real conversion steadily increased with time up to 120 min, the longest time tested. The slowdown in the reaction rate observed after 60 min was probably due to loss of sample reactivity as observed in previous work.2,6,7 By contrast, the apparent conversion showed a marked reduction between 0 and 10 min to continuously increase thereafter. The amount of carbonaceous material deposited on top of the precoated deposits on the catalyst during the hydrocracking is also shown in Table 1. Despite the precoating, significant new deposition of carbonaceous material took place on the catalyst during the 0 min run. The amount of deposits sharply decreased within the first 10 min of reaction (at temperature), clearly indicating redissolution of the carbonaceous layer, thereafter presenting slower variations between 10 and 60 min. The negative additional carbonaceous deposition shown in Table 1 for the 30 min run could be due to either experimental error or a small net dissolution of precoating material on the catalyst taking place during that experiment. Although there was an increase in the amount of carbonaceous deposit between 60 and 120 min, the weight change was significantly slower than during the initial deposition. The trend followed by the apparent conversion was the result of two different processes simultaneously occurring in the reactor. First, longer reaction times allowed hydrocracking to proceed to greater extents, producing reductions in the >450 °C boiling fraction. On the other hand, carbonaceous material deposited on the catalyst during heatup partially redissolved in

the reaction mixture, producing a diminution in the total amount of carbonaceous deposit (Table 1). The deposits on the catalyst tend to preferentially form from heavy fractions,1,18 and therefore, their dissolution was likely to generate an increase in the amount of >450 °C material in solution. Although some of it may have reacted on the catalyst and redissolved as lighter materials, this cannot be told by the data. On the other hand, the increase in the >450 °C fraction accompanying the dissolution of deposits is evident. Within the first 10 min, the dissolution rate of the deposits was higher than the hydrocracking rate and consequently the apparent conversion dropped. After 10 min, both apparent and real conversion increased. Although differences between conversions at 0 and 5 min were within experimental error ((3 in real conversion data), they were consistent with the process described above and showed a drop in apparent conversion while the real conversion rose. Real conversions are not affected by the redissolution of >450 °C material since the deposits were already taken as part of the >450 °C fraction. These parallel events, hydrocracking and redissolution of the deposits, explain the difference in behavior followed by “apparent” and “real” conversions. 3.3. Size Exclusion Chromatography of the Products. SEC chromatograms of the Point of Ayr “hydrocracker feed” and its hydrocracking products are shown in Figures 2 and 3. Similar trends have been recorded at all wavelengths. Only results from detection at 350 nm are shown. These data refer to liquid products and therefore exclude material deposited on the catalyst. All chromatograms exhibited two peaks: one showing material resolved by the column porosity and the other corresponding to material excluded from it. This is in line with the pattern generally shown by coal- and petroleum-derived heavy hydrocarbon liquids.15,19-24 (18) Suelves, I.; Lazaro, M. J.; Begon, V.; Morgan, T. J.; Herod, A. A.; Kandiyoti, R. Fractionation of coal extracts prior to hydrocracking: an attempt to link sample structure to conversion levels and catalyst fouling. Energy Fuels 2001, 15, 1153-1165. (19) Begon, V.; Suelves, I.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Structural effects of sample ageing in hydrocracked coal liquefaction extracts. Fuel 2000, 79, 1423-1429. (20) Suelves, I.; Islas, C. A.; Herod, A. A.; Kandiyoti, R. Comparison of fractionation methods for the structural characterization of petroleum asphaltenes. Energy Fuels 2001, 15, 429-437.

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Figure 2. Mixed-A column SEC chromatograms of the Point of Ayr hydrocracking feed and the hydrocracking products for 0, 5, and 10 min reaction time. All chromatograms have been area normalized.

Figure 3. Mixed-A column SEC chromatograms of the Point of Ayr hydrocracking feed and the hydrocracking products for 5, 30, and 60 min reaction time. All chromatograms have been area normalized.

All products showed a smaller excluded (large mass) peak than that of the feed. The size of this peak decreased with reaction time. A shift of the high mass end of the retained peak toward longer elution times was also observed with a progres(21) Suelves, I.; Islas, C. A.; Millan, M.; Galmes, C.; Carter, J. F.; Herod, A. A.; Kandiyoti, R. Chromatographic separations enabling the structural characterisation of heavy petroleum residues. Fuel 2003, 82, 1-14. (22) Herod, A. A.; Lazaro, M. J.; Suelves, I.; Dubau, C.; Richaud, R.; Shearman, J.; Card, J.; Jones, A. R.; Domin, M.; Kandiyoti, R. Size exclusion chromatography of soots and coal-derived materials with 1-methyl-2-pyrrolidinone as eluent: observations on high molecular mass material. Energy Fuels 2000, 14, 1009-1020. (23) Herod, A. A.; Shearman, J.; Lazaro, M. J.; Johnson, B. R.; Bartle, K. D.; Kandiyoti, R. Effect of LiBr addition to 1-methyl-2-pyrrolidinone in the size-exclusion chromatography of coal-derived materials. Energy Fuels 1998, 12, 174-182. (24) Zhang, S.-f.; Xu, B.; Herod, A. A.; Kimber, G. M.; Dugwell, D. R.; Kandiyoti, R. Effect of coal rank on hydrocracking reactivities of primary coal extracts prepared in a flowing-solvent reactor. Fuel 1996, 75, 15571567.

sion in time in the series feed, 0 and 10 (these two were almost undistinguishable among themselves), 30, and 60 min. These changes in the SEC chromatograms showed the expected overall reduction in molecular size with reaction time. An exception to this general trend was found in the 5 min run. The SEC chromatogram of the 5 min products showed a considerable shift of the retained peak toward longer elution times than that of the 0 min run. The elution profile of this run is similar to that observed at 60 min. Retrogressive behavior was observed between 5 and 10 min, with a shift of the retained peak toward shorter retention times and an increase in the proportion of excluded material. Two distinct processes are thought to have led to these results. The first is the natural progression in the breakup of larger molecules in the reactive environment within the reactor. Meanwhile, between 0 and 5 min, a large proportion of the heavier material originally contained in the feedstock was found



Figure 4. Synchronous UV-fluorescence spectra of the Point of Ayr hydrocracking feed and the hydrocracking products for 0, 5, and 10 min reaction time. All spectra have been peak normalized.

Figure 5. Synchronous UV-fluorescence spectra of the Point of Ayr hydrocracking feed and the hydrocracking products for 5, 30, and 60 min reaction time. All spectra have been peak normalized.

deposited on catalyst particles (Table 1). Between 5 and 10 min, much of the freshly deposited material is observed to dissolve back in the increasingly lighter liquid mixture. As mentioned above in the discussion on conversions, deposits on the catalyst surfaces tend to be preferentially from the heavier materials and their redissolution would be likely to shift the SEC chromatogram toward larger molecular sizes. During these developments, measured real conversions of >450 °C boiling material would not be immediately affected by the redissolution of >450 °C boiling material since the deposits were already taken as part of the >450 °C boiling fraction. These parallel events involving the hydrocracking of the overall sample mass and the redissolution of the deposits go some way toward explaining differences in behavior shown by SEC chromatograms and the two conversions. In addition, it must be noted that not all material in the >450 °C boiling fraction is expected to appear as “large” in SEC, since that

fraction can also contain relatively small molecules such as benzopyrene. Therefore, a total correlation between SEC and conversion data is not expected. 3.4. UV-Fluorescence Spectrometry. The UV-fluorescence (UV-F) spectra of the Point of Ayr hydrocracker feed and its hydrocracked products are shown in Figures 4 and 5. The Point of Ayr hydrocracker feed showed signal at longer wavelengths than the hydrocracked products, indicating that higher concentrations of larger polynuclear aromatic rings are present in the feed. The spectra of the 0, 10, 30, and 60 min runs were almost undistinguishable. The spectrum of the 5 min run product was clearly shifted toward shorter wavelengths. This suggests the presence of (on average) smaller aromatic rings in solution than those observed in the rest of the samples. Between 5 and 10 min, the spectrum shifted toward longer wavelengths as a consequence of the dissolution of material which was deposited


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Table 2. BET Specific Surface Area, Total Pore Volume, and Average Pore Diameter of Fresh and Coated NiMo/Al2O3 and the Catalysts Recovered from Hydrocracking Runs sample

BET specific surface area (m2/g)

tot pore vol (cm3/g)

avg pore diam (nm)

fresh NiMo/Al2O3 coated NiMo/Al2O3 0 min run 10 min run 30 min run

78 45 51 52 51

0.28 0.12 0.09 0.13 0.12

14.7 11.0 7.3 10.2 9.4

on the catalyst. This behavior is consistent with that shown by the SEC chromatograms of the liquid products, as discussed above. 3.5. Specific Surface Area Measurements. Specific surface area measurements were carried out on fresh NiMo/Al2O3 catalyst, the coated NiMo/Al2O3 catalyst used as feed in these experiments, and the catalysts recovered from the 0, 10, and 30 min runs (Table 2). Table 2 presents a summary of the results. A significant decrease in specific surface area takes place during the catalyst coating process. A slight increase in specific surface area was observed after the 0 min reaction time run (i.e., heatup only). The variations in specific surface area observed thereafter were found to be small. So whatever catalyst activity was observed could not be attributed to increased specific surface areas, per se. The changes in pore volume and average pore diameter followed the changes in carbonaceous content of the catalyst. Both parameters showed a marked decrease due to the coating process and the 0 min reaction (i.e., during heatup), in line with the large deposition taking place during the initial stages. However, this initial heavy deposition was clearly reversible. Increases in pore volume and pore diameter were seen for the 10 min run, in agreement with the reduction in deposits observed by TGA. 4. Discussion The extent of hydrocracking showed a continuous progression, as expressed by the trend of the real conversion, between 0 and 120 min. This marks a difference with previous work,7 which showed a drop in conversion during the first 10 min of reaction. Data published in that work did not consider the

carbonaceous deposits as part of the >450 °C boiling fraction of the products in the calculation of conversion. This previous method of calculation did not therefore allow distinguishing between (i) reduction in the >450 °C boiling fraction due to hydrocracking and (ii) removal of these materials from the liquid products as deposits on the catalyst. The need for differentiating between these two processes has led to the definition of real conversion, as shown in the present work. In industrial processes, the amount of material deposited on the catalyst is expected to be negligible in comparison with the amount of feed treated, and therefore, both conversions would be very similar. However, in experiments conducted with small amounts of sample, this difference is relevant and the fraction of the feed forming deposits must be taken into account in order to evaluate a catalyst. The real conversions, including the carbonaceous deposits as part of the >450 °C boiling fraction, have been recalculated here using data from ref 8. Both conversions are plotted in Figure 6 as a function of reaction time. Apparent conversion refers to that published in ref 7, whereas real conversion is the recalculated one, following the definitions presented in the Experimental Section (above). There are two basic differences between the two works: (i) Tetralin was used as a donor-solvent. (ii) The catalyst was not coated with carbonaceous material. In Figure 6, the drop in conversion at short reaction times was previously explained in terms of a period needed to achieve the activation of the catalyst by sulfidation.7 The fragments produced by thermal cracking of the sample at short reaction times were not hydrogenated due to the absence of an active catalyst and recombined to form larger molecules. As shown in Figure 6, the trends followed by the conversion calculated in both ways are identical, and to that extent, the conclusions of that work remain valid. The use in this work of an already coated catalyst, which has been sulfided during the coating process, eliminated this activation period, and therefore, the real conversion was observed to increase monotonically. Extensive deposition on fresh catalyst in the early stages of the hydrocracking process has been widely reported in the literature.7,10,11,25,26 The present work shows that a relatively large deposition also takes place (in the presence of fresh feed)

Figure 6. Apparent and real conversions as functions of reaction time obtained by Begon et al.7



Figure 7. Apparent and real conversions as a function of reaction time obtained in the present work.

even when a previously coated catalyst with a high carbonaceous content (30%) is used. This suggests that a fraction of the feed has a strong tendency to adsorb on the solid surface provided by the catalyst. These “fresh” deposits seem to quickly redissolve in the liquid mixture once the full reaction temperature is reached. The redissolution process is relatively slow during the first 5 min, but it speeds up between 5 and 10 min. As a result, a weight equivalent to all the material initially deposited on the catalyst was transferred into the solution between 5 and 10 min. Clearly, the carbonaceous deposits are dynamic and material is exchanged between them and the solution. The dynamic nature of the deposits probably contributes to explaining the sustained activity of the catalyst despite its extensive carbonaceous coating. This bulk exchange of material between the deposits and the solution could be additional to other mechanisms suggested in earlier work.27 It had been proposed there that the carbonaceous deposits might act as a permeable layer, allowing the exchange of hydrogen between the liquid phase and the molecules adsorbed on the catalyst surface. The curves of apparent and real conversion as a function of time are shown in Figure 7. The increase in real conversion slowed down in the second hour of hydrocracking due to the completion of the faster reactions with the subsequent loss of reactivity of the feedstock. Despite the use of a precoated catalyst, similar amounts of material deposited on the catalyst as those reported by Begon et al.7 on the fresh catalyst were observed. The absence of tetralin in the reactor made the feed more viscous, which may have had an effect on this observation. 5. Summary and Conclusions The deactivation of a NiMo/Al2O3 catalyst in the early stages of hydrocracking of a heavy hydrocarbon liquid has been studied. The use of an already coated catalyst and absence of a donor-solvent (e.g.) tetralin as solvent were two key elements in the experiments that have been described. Instead, the (25) Ancheyta, J.; Betancourt, G.; Centeno, G.; Marroquin, G.; Alonso, F.; Garciafigueroa, E. Catalyst Deactivation during Hydroprocessing of Maya Heavy Crude Oil. 1. Evaluation at Constant Operating Conditions. Energy Fuels 2002, 16, 1438-1443. (26) Ancheyta, J.; Betancourt, G.; Centeno, G.; Marroquin, G. Catalyst Deactivation during Hydroprocessing of Maya Heavy Crude Oil. II. Effect of Temperature during Time-on-Stream. Energy Fuels 2003, 17, 462-467.

experiment relied on the donor ability of the recycle solvent which had been a feature of the Point of Ayr pilot plant.12 The absence of an externally added donor-solvent (tetralin) in the reaction mixture enabled product characterization to be carried out without interference from tetralin derivatives. Meaningful boiling point distributions can now also be obtained by TGA in the region of boiling points below 450 °C provided the weight loaded on the TGA pan can be precisely controlled. The use of the precoated catalyst in a batch reactor went some way toward simulating a continuous process where fresh feed is continuously added onto already carbon-coated catalyst. Employing a batch reactor enabled the observation of the redissolution of the initial carbonaceous deposits into the bulk liquid. This redissolution process is not observable in continuous reactors due to the steady supply of heavy materials in the feed. Although a precoated catalyst was used, the initial sample deposition on solid surfaces was large. It appears that a fraction of the feed has a strong tendency to form deposits. However, the carbonaceous layer coating the catalyst was observed to be dynamic and exchanged material with the solution. The amount of deposits rapidly decreased due to redissolution of much of the coating material. This bulk exchange of material between the deposits and the solution is a mechanism which contributes to explaining the sustained level of catalytic activity despite the large carbonaceous deposition. However, this mechanism is not exclusive of others proposed in earlier works, such as the permeability of the deposits to hydrogen, which would enable the hydrogenation of molecules on the catalyst surface. The initial deposition caused some degree of pore blockage which led to a decrease in both the total pore volume and the average pore diameter of the catalyst. The subsequent dissolution of a fraction of the carbonaceous layer tended to partially reverse the pore blockage and allowed total pore volume and average pore diameter to increase. On the other hand, the specific surface area did not present significant variation with hydrocracking time. These changes in the amount of deposits affected the apparent conversions and the SEC and UV-F profiles obtained within the first 10 min. SEC and UV-F showed the presence of larger molecules in solution at 10 min in comparison with the 5 min (27) Thomson, S. J.; Webb, G. Catalytic hydrogenation of olefins on metals: a new interpretation. J. Chem. Soc., Chem. Commun. 1976, 13, 526-527.


products as a result of the redissolution of carbonaceous deposits. In addition, the apparent conversion, which is calculated without taking the deposits into account as carbonaceous material, dropped in the first 10 min. However, adding the deposited material to the >450 °C boiling material (in the conversion calculation), conversions were found to steadily increase over the 120 min period considered. This marks a difference with the trend followed by conversion as a function of reaction time

Millan et al.

in earlier experiments. This difference has been related to the use of a precoated catalyst in the present study and, therefore, to the absence of the catalyst activation period by sulfidation observed previously. Acknowledgment. The support from EPSRC under Grant GR/ R27471/01 is gratefully acknowledged. EF060466O

Effect of Catalyst Deactivation and Reaction Time on Hydrocracking

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