Energy & Fuels 2006, 20, 45-53
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Accelerated Solvent Extraction of Spent Hydrotreating Catalysts: A Study on Oil and Coke Fractions Fatima A. Ali,* Andre’ Hauser, Hanadi. A. Abdullah, and Awatef Al-Adwani Central Analytical Laboratory, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat, 13109, Kuwait ReceiVed June 27, 2005. ReVised Manuscript ReceiVed NoVember 9, 2005
Catalyst deactivation by coke deposition is a problem of great and continuous concern in residue hydroprocessing operations. To reach a better understanding of the very complex nature of coke it is desirable to obtain information on the amount and composition of both the insoluble (IS) and soluble (S) fractions of coke. A standard procedure, which could be adapted by all laboratories involved in investigation of spent catalysts, would make the results obtained more meaningful. Accelerated solvent extraction in comparison with Soxhlet extraction was evaluated concerning the efficiency and relevancy (recovery, experiment time and volume of solvent used) for coke extraction. The accelerated solvent extraction reached the same efficency as Soxhlet after 5 min of extraction compared with 6-12 h by using less solvent. Four solvent-defined coke fractions, such as heptane- (HEP-S), toluene- (TOL-S), tetrahydrofuran- (THF-S), and dichloromethanesoluble (DCM-S) coke, were obtained from spent catalysts used in atmospheric residue hydroprocessing. For a start of run catalyst (1 h-240 h) the S-coke decreases rapidly from 75% (1 h) to 30% (240 h). The sequence of yields is THF-S (external coke precursors) < TOL-S (asphaltenes) < DCM-S (internal coke precursors). The catalysts from the end of run contained about 30 wt % HEP-S coke (carryover of feed and product). Depending on the location of the catalyst in the hyrdrotreating unit, TOL-S coke amounted 3-10 wt %. THF-S and DCM-S coke made up to 10 wt % of the total carbonaceous deposit. Temperature-programmed oxidation and 13C nuclear magnetic resonance were used to study the nature of the soluble as well as the insoluble coke fractions.
Introduction Coke formation is a major concern in refining industry. Beside reactor fouling and pipe clogging, coke build up is one of the main routs for catalyst deactivation especially during residue upgrading.1,2 Therefore, a great interest is taken in the elucidation of the mechanism of coke formation as well as in the chemical structure of the coke components. Coke has a very complex nature and is often described as a highly aromatic and hydrogen-deficient carbonaceous deposit.3 For example, coke deposits occurring in hydrodesulfurization of residue have been classified as: (i) reversibly adsorbed parts of the feed and product, (ii) reversibly adsorbed coke precursors (e.g., asphaltenes), and (iii) irreversibly adsorbed polyaromatics, which form clusters and eventually a crystalline phase with increasing reaction time.3 The latter, so-called hard coke, causes severe catalyst deactivation. Both reversible and irreversible adsorbed cokes are formed right from the start of run (SoR) with a high rate.4 After initial coking, the rate reduces remarkably, and during the middle of run (MoR) coke formation is slow until at the end of run (EoR) a very rapid deactivation is observed. The final loss in activity is attributed to an ultimate pore blockage by coke and metals.5 * To whom correspondence should be addressed. Phone: ++9654836100. Fax: ++965-4815197. E-mail:
[email protected]. (1) Furimsky, E.; Massoth, F. E. Catal. Today 1999, 52, 381-495. (2) Bartholomew, C. H. Appl. Catal., A 2001, 212, 17-60. (3) Beuther, H.; Larson, O.A.; Perrotta, A. J. Stud. Surf. Sci. Catal. 1980, 6, 271-282. (4) Matsushita, K.; Hauser, A.; Marafi, A.; Koide, R.; Stanislaus, A. Fuel 2004, 83, 1031-1038.
One of the limitations in the determination of the chemical identity of the coke components is the complexity of the material. The only way of obtaining more detailed information on the chemical nature of coke is its fractionation by gradual solubilization.6 The amount and composition of the soluble portions of the carbonaceous deposit depends on the solvent used, the extraction technique, the aging conditions, catalyst type, and the feedstock used. A standard procedure, which could be adapted by all laboratories involved in coke analysis, would be desirable in order to make results comparable. In view of the above, a study was undertaken to evaluate two extraction methods, namely, conventional Soxhlet extraction7 and accelerated solvent extraction (ASE),8,9 with regard to their efficiency and relevancy for characterization of coke on industrially used catalysts obtained from pilot plant experiments in our laboratories and industrial atmospheric residue desulfurization (ARDS) units. Experimental Section Catalysts. Three industrially applied catalysts, namely, a hydrodemetallization (HDM) Mo/Al2O3 (A), a hydrodesufurization (HDS) Ni/Mo/Al2O3 (B), and a hydrodesufurization/-denitogenatoin (5) Hannerup, P.; Jacobson, A. C. Prepr. Am.sChem. Soc., Pet. Chem. DiV. 1983, 28, 576-583. (6) Storm, D. A.; Decanio, S. J.; Edwards, J. C.; Shue, E. Y. Pet. Sci. Technol. 1997, 15, 77-102. (7) Van Doorn, J.; Moulijn, J. A. Fuel Proc. Technol. 1990, 26, 39-51. (8) Kenny, D. V.; Olesik, S. V. J. Chrom. Sci., 1998, 36, 59-65, 6672. (9) Mochida, I.; Zhao, X.; Sakanishi, K.; Yamamoto, S.; Takashima, H.; Uemura, S. Ind. Eng. Chem. Res. 1989, 28, 418-421.
10.1021/ef0501905 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/14/2005
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Table 1. Characteristics of Catalysts A, B, and C Used in the Present Study29 catalyst characteristics catalyst type bulk density surface area average pore diameter active metal metal content Mo Ni P metal capacity
unit
A
B
C
g/mL m2/g Å
HDM 0.4-0.5 150-200 150-200 Mo
HDS 0.6-0.7 200-250 80-100 Ni, Mo
HDS/N 0.7-0.8 170-200 80-100 Ni, Mo, P
2-3
7-9 2-3
high
medium
9-11 2-4 2-4 low
wt % wt % wt %
(HDS/N) Ni/Mo/P/Al2O3 (C) catalyst, used in refineries in the front-, middle-, and back-end beds, respectively, of an ARDS unit, were used in the study. The characteristics of the fresh catalysts are presented in Table 1. Apparatus. A Dionex (Sunnyvale, CA) accelerated solvent extractor ASE-300 system was used as an extractor. A high-pressure vessel, having a volume of 34 mL, was used as an extraction cell. The system uses liquid solvents at elevated temperatures and pressures to increase the efficiency of the extraction process. Parallel, a Soxhlet extraction apparatus (KIMAX 45/50, 24/40) was used to extract the same catalysts at ambient pressure and elevated temperature according to the boiling points of the solvent (see Table 1). Materials. All solvents, namely, n-heptane (HEP, bp 98.4 °C, polarity10 0.012), toluene (TOL, 110.4 °C, 0.099), tetrahydrofuran (THF, 66.0 °C, 0.207), dichloromethan (DCM, 39.8 °C, 0.309), pyridine (PYR, 115 °C, 0.523), and methanol (MeOH, 64.6 °C, 0.762), used in our study were analytical grade. Cellulose thimbles from Supelco (cat no. 64840-U) were used for Soxhlet extraction. Extraction Procedures. ASE. Approximately 10 g of catalyst was packed in a 34-mL extraction cell between two glass-fiber filters (see Figure 1). Each catalyst was sequentially subjected to different solvents (HEP, TOL, THF) and alternatively to DCM, PYR, or MeOH. The extract was collected in a receiving bottle (200 mL). The extraction was carried out with two static times (5 and 60 min) and at different temperatures (120, 160, 180, 200 °C). The other operating parameters were: pressure, 1,500 psi; flush volume, 60% of cell volume; two extraction cycles. Soxhlet Extraction. About 10 g of catalyst was Soxhlet extracted in a conventional Soxhlet apparatus using the same six solvents as used for ASE. The catalyst was placed in a cellulose thimble, which fit in the Soxhlet apparatus. The extraction was terminated after no further discoloration of the solvent was observed. The temperature of extraction depended on the boiling point of the extraction solvent, the actual extraction temperature being lower than the boiling point. Before and after each extraction step, the catalyst was completely dried and weighed. About 1 g of the catalysts was retained for further analyses. SolVent EVaporation. The solvent was removed from the extract under vacuum using a rotary evaporator up to a volume of about 5 mL. The remaining solvent was removed in a vacuum oven (NU ¨ VE, EV018) at 75° C and -1.0 bar. The weight of the residue was recorded. Analytical Methods. Elemental analysis (EA) was carried out using an elemental analyzer (i.e., CE Instruments model EA1110 CHNS). Each sample was analyzed in duplicates. An energy-dispersive X-ray (XRF) spectrometer from Shimadzu (EDX-700) was used to determine the Ni and V contents on the spent and extracted catalysts. The instrument was calibrated using standards for both individual metals. Thermogravimetric analysis (TGA) was carried out on a SHIMADZU TGA-50 by using approximately 20 mg of the spent and (10) Reichardt, Ch. SolVents and SolVent Effects in Organic Chemistry, 2nd ed.; VCH Publishers: 1988.
Figure 1. Filling of ASE extraction cell.
extracted catalysts to determine the moisture content of the samples. The analyses were run in air atmosphere with a flow rate of 40 mL/min. The heating program was: temperature range, 30-130ο C; heating rate, 10ο C/min; holding time at 130ο C, 120 min. Temperature-programmed oxidation (TPO) analysis was conducted on the spent and extracted catalysts using a thermogravimetric analyzer equipped with a mass (MS) detector (TGA, Mettler Toledo model SDTA851e; MS, Balzers model Thermostar). About 20 mg of the catalyst sample was heated in the presence of a gas mixture containing 15% O2 in He flowing at a rate of 60 mL/min. The different gaseous products (CO2, SO2, NO2) generated were monitored as a function of temperature using a mass detector. A linear heating rate of 10 °C/min was used throughout the analyses. Nuclear magnetic resonance (NMR) were used to investigate the spent catalysts, extracted catalysts, as well as their extracts applying solid-state and liquid-state NMR, respectively, using a 300 MHz (7.0463 T) spectrometer from Bruker (AVANCE 300) equipped with an automated pneumatic unit for magic angle spinning (MAS). Solid-State SPE/MAS 13C NMR. The single-pulse excitation (SPE) spectra of the spent and extracted catalysts were obtained from ground samples using a 4-mm multinuclear probe. The MAS-rate was 13 kHz to minimize overlapping between the main signals (aliphatic; 0-70 ppm and aromatic carbon 90-200 ppm) and the spinning sidebands. A pulse length of 2.5 µs was used corresponding to the 45° 13C flip angle. The protons were inversely gated decoupled with maximum power (120 W). The recycling delay was 10 s. All spectra were measured with a sweep width of 34 kHz, and the free induction decay was sampled with 4 k data points. Liquid-State NMR. 1H measurements were carried out with a spectral sweep width of 4.5 kHz, a pulse angle of 18 µs (90°), and a delay time of 3 s. Parameters for 13C inverse gated decoupling measurements were: spectral widths, 20 kHz; pulse widths, 6 µs (30°). Samples for 1H NMR measurements were prepared by adding 0.5 mL of methylenechloride-d2 (CD2Cl2) to ca. 10 mg of the extract in a 5-mm tube. Tetramethylsilane (TMS) was used as an internal reference (0 ppm). For 13C measurement, 1.5 mL of deuteriochloroform (CDCl3) was added to ca. 200 mg of the extract in a 10mm tube. The spectra were referenced to CDCl3 at 77.7 ppm.
Results and Discussion Comparison Between Soxhlet Extraction and ASE. The extraction experiments aimed at the evaluation of both extraction techniques, Soxhlet and ASE, with regard to their efficiency and relevancy for characterization of coke on spent catalysts obtained from pilot plant experiments in our laboratories and from industrial ARDS units. Compared to the a huge number of publications about coke formation on hydrotreating catalysts there are only few papers about coke fractionation by graded solubilization of coke or cokelike material using different solvents, such as alkanes,6 aromatics,6,7,11,12 tetrahydrofuran,11 (11) Richter, I. E.; Jones, B. A.; Ezzel, J. L.; Porter, N. L. Anal. Chem. 1996, 68, 1033-1039. (12) Gy, M. R.; Zhao, Y.; McKnight, C. M. Fuel 2000, 285-294.
SolVent Extraction of Spent Hydrotreating Catalysts
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Table 2. Yield of Solvent-Defined Coke Fractions Obtained from Spent Catalysts of Type C: Comparison between Soxhlet and ASE Solvent-defined coke fractionsa method
HEP-Sb
TOL-Sb
THF-Sb
DCM-Sb
sum
Soxhlet ASE/120c ASE/160c ASE/180c ASE/200c
33.5% 35.1% 31.5% 34.0% 31.5%
1.4% 2.3% 0.9% 1.0% 0.5%
7.3% 1.2% 6.9% 7.0% 6.5%
0.1% 0.1% 0.5% 2.3% 4.1%
42.3% 38.7% 39.8% 44.3% 42.6%
a Total coke on catalyst equals 100%. b HEP-S, TOL-S, THF-S, and DCM-S stand for soluble coke fractions according to the solvent used. c 200, 180, 160, and 120 stand for the extraction temperature in °C.
halogenated hydrocarbons,7,13-15 and heteroaromatic solvents.12 The solvents, mainly used in this study, differ in their polarity and ability to dissolve hydrocarbons that are constituents of petroleum residues or carbonaceous deposits. In this section the percentage of soluble (S) and insoluble (IS) coke vs the solvent used and the method applied is presented and discussed. Table 2 shows the percentage of solvent defined coke fractions obtained from a spent catalyst of type C used in an industrial ARDS unit. This catalyst contained 75 g of coke per 100 g of fresh catalyst. By comparison of the yields of total extractable matter (last column in Table 2), it becomes obvious that ASE at g180 °C is as efficient as Soxhlet extraction. In ASE, with increasing temperature an increase in the extraction yield is observed. As for the individual solvents, however, it seems that ASE with HEP above 120 °C already removes parts of the toluene soluble (TOL-S) coke and its yield reduces in the extraction cycle following the HEP extraction as observed in our experiments. This fact suggests that a mild extraction temperature is advantageous for the first two extraction cycles (HEP, TOL) to achieve a better fractionation according to the chemical nature of the soluble coke components (compare ASE/200 and ASE/120 in Table 2). DCM and THF extraction, however, requires high temperature to optimize the fraction yields of THF-S and DCM-S coke, respectively. Additional to DCM, pyridine (PYR), and methanol (MeOH) were tested as solvents in the finishing extraction step. These two solvents, however, are unsuitable under the experimental conditions applied since it is, on one hand, almost impossible to remove PYR from the extracted catalyst as well as the PRY-S coke fraction12 and on the other hand MeOH reacts at a pressure of 1500 psi and elevated temperature with the coke, the catalyst support, and the MeOH-S coke fraction. As Figure 2 demonstrates, the extraction yield in the finishing step (DCM-S coke) can be increased by a factor of 40 by increasing the temperature from 120 to 200 °C (Figure 2). For further studies on a variety of spent catalysts obtained from pilot plant experiments as well as from an industrial ARDS unit we have chosen the following ASE conditions: temperature, 120-200 °C; pressure, 1500 psi; flush solvent volume, 41 mL; extraction time, 5 min; extraction cycles, 2. In the course of this study, two kinds of coke, such from the SoR and EoR, were fractionated by ASE using four solvents (HEP, TOL, THF, DCM). SoR Coke. For a SoR coke on a catalyst of type A obtained from a pilot plant experiment running over 240 h, Figure 3 (13) Kozhevnikov, I.; Holmes, S.; Siddiqui, M. R. H. Appl. Catal., A 2001, 214, 47-58. (14) Holmes, S. M.; Garforth, A. G.; Maunders, B.; Dwyer, J. Appl. Catal., A 1997, 151, 355-372. (15) Chu, K. S.; Dong, D.; Hanson, F. V.; Massoth, F. E. Ind. Eng. Chem. Res. 1996, 35, 4012-4019.
Figure 2. Yield of DCM-S coke vs extraction temperature for ASE on a spent catalyst of type C.
shows the yields recovered after extraction with three solvents, such as TOL, THF, DCM. Before unloading the catalysts from the pilot plant, the rector was purged with gasoline to remove feed carryovers. The IS coke increases from 25 to 70 wt % if the coke is aged between 1 and 240 h. Throughout all spent catalysts, the highest yield was achieved with THF and the lowest with DCM. EoR Coke. For the EoR coke, two catalyst systems, one coming from a pilot plant experiment (I) and another one stemming from an industrial ARDS unit (II), were extracted by ASE using four different solvents (HEP, TOL, THF, DCM). System I, consisting of all three catalyst types A, B, and C, were coked for more than 6000 h of time-on-stream (TOS) and washed with gasoline before unloading them from the reactors. Figure 4 compiles the yields of soluble coke collected after ASE on the three spent catalysts. The soluble coke ranges between ∼9 wt % (B) to ∼4 wt % (A, C), what shows, that compared to the SoR, most of the coke at the EoR is very refractory and insoluble even under severe extraction conditions such as high temperature and pressure. The catalysts system II consisting of type B and C catalysts were aged in an industrial ARDS unit for more than 6500 h TOS. These catalysts carried an appreciable quantity of carryovers from the feed and product oils (HEP-S) that were dissolved in heptane (see Figure 5). After removal of carryovers, about 22 wt % (B) and 13 wt % (C) of the total carbonaceous depositions on the catalysts were soluble. These yields are clearly higher than those collected for the same types of catalysts aged in a pilot plant unit. To characterize the chemical nature of the soluble (TOL-S, THF-S, DCM-S) and insoluble coke fractions (HEP-IS, TOL-IS, THF-IS, DCM-IS) both were subjected to a number of analyses. Coke Characterization. TPO. TPO was conducted for the extracted spent catalysts carrying both types of coke, such from SoR and EoR. The major gaseous products, namely, CO2, SO2, and NO2, were detected by mass spectroscopy (MS), and the relative intensity of the MS signal vs combustion temperature is shown in Figures 6 and 7. SoR Coke. Figure 6 displays the TPO profiles of CO2, SO2, and NO2 emitted from SoR coke (TOS, 48 h) deposited on an A-type catalyst. The CO2 profiles show two overlapping peaks, with one maximum at 350 °C and another one at about 450 °C. In light of the short-term coking, the two fractions of coke can be classified as a reactive soft coke, which can be oxidized more easily, and as a refractory surface coke, which is strongly adsorbed at the catalyst support.16-18 Several authors16,19 have (16) Marafi, M.; Stanislaus, A. Appl. Catal., A 1997, 159, 259-267.
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Figure 3. Yields of S and IS coke fractions obtained from a spent catalyst of type A vs TOS.
Figure 4. Yields of S coke fractions obtained from the catalyst system I (pilot plant unit) at the EoR for catalyst types A, B, and C.
reported similar TPO profiles for hydrotreating catalysts, which were aged over a short period of time (TOS, 0.5-190 h). The NO2 and SO2 profiles indicate that the formation of the two gases occurred at the same temperatures showing three distinct peak maxima, one below 200 °C (clearly seen in Figure 6c), one at ∼300 °C and another one at ∼450 °C. The latter two coincide with the CO2 peaks. The SO2 peak
