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Energy & Fuels 2006, 20, 320-323
Optimal Conditions for Coke Extraction of Spent Catalyst by Accelerated Solvent Extraction Compared to Soxhlet Hanadi A. Abdullah, Andre´ Hauser, Fatima A. Ali,* and Awatef Al-Adwani Central Analytical Laboratory, Kuwait Institute for Scientific Research, P.O. Box 24885, Safat, 13109 Kuwait ReceiVed July 23, 2005. ReVised Manuscript ReceiVed NoVember 16, 2005
Catalyst deactivation by coke deposition is a problem of great and continuing concern in residue hydroprocessing operations. To reach a better understanding of coke formation, it is desirable to obtain information on the amount and composition of soluble fractions of coke deposited. Since extraction of spent catalysts by a number of solvents with gradually increasing polarity is time-consuming, this study was undertaken to evaluate two extraction methodssSoxhlet extraction and accelerated solvent extraction (ASE)swith regard to their efficiency and relevancy (recovery, experiment time, and volume of solvent used) for characterization of coke. In both methods, spent catalysts were successively extracted with n-heptane, toluene, tetrahydrofuran, and dichloromethane. 13C nuclear magnetic resonance (NMR) was used to study both the soluble coke and the insoluble coke on the catalysts. ASE was found to be the superior extraction method because ASE reached the same efficency as Soxhlet after 5 min of extraction compared with 6-12 h by Soxhlet using less solvent.
1. Introduction Catalyst deactivation in the hydroprocessing of heavy feedstocks is a major concern in the refining industry. In addition to reactor fouling and pipe clogging, coke buildup is one of the main routes for catalyst deactivation, especially during residue upgrading.1,2 The final loss in activity is attributed to an ultimate pore blockage by coke and metals,3 but the causes of coke formation and catalyst deactivation are not fully understood. Therefore, several studies were carried out on the elucidation of the mechanism of coke formation as well as of the chemical structure of the coke components. The only way of obtaining more detailed information on the chemical nature of coke is its fractionation by gradual solubilization.4 The amount and composition of the soluble portions of the carbonaceous deposit depend on the solvent used, the extraction method, the aging conditions, the catalyst type, and the feedstock used. A standard extraction procedure that could be adapted by all laboratories involved in coke analysis would be desirable in order to make results comparable. The solvents that have been commonly used in such extractions are heptane (HEP), toluene (TOL), tetrahydrofuran (THF), pyridine (PYR), methanol (MeOH), and dichloromethane (DCM). To accomplish coke extraction using Soxhlet, a period of not less than 4 h is needed for each solvent, which makes the extraction a very time-consuming step in coke characterization. A new technique was tried to speed up the extractionsthe accelerated solvent extraction (ASE), which has been applied mainly to environmental and food samples under variable conditions (temperature, contact time, and flush volume).5 * Corresponding author. Phone: +965 483 6100. Fax: +965 481 5197. E-mail:
[email protected]. (1) Furimsky, E.; Massoth, F. E. Catal. Today 1999, 52, 381-495. (2) Bartholomew, C. H. Appl. Catal. A: Gen. 2001, 212, 17-60. (3) Hannerup, P.; Jacobson, A. C. Prepr. Pap.sAm. Chem. Soc., DiV. Pet. Chem. 1983, 28, 576-583. (4) Storm, D. A.; Decanio, S. J.; Edwards, J. C.; Shue, E. Y. Pet. Sci. Technol. 1997, 15, 77-102.
Additionally, ASE allows an automated operation using an auto sampler and a solvent controller, so that up to 12 samples can be sequentially extracted without manual interference. In view of the above, a study was undertaken to evaluate two extraction methods, Soxhlet extraction6 and ASE,7,8 with regard to their efficiency and relevancy for characterization of coke on industrially used catalysts obtained from atmospheric residue desulfurization (ARDS) units. 2. Experimental Section 2.1. Catalysts. Three industrially applied catalysts, namely, a hydrodemetallization (HDM) Mo/Al2O3 (A), a hydrodesufurization (HDS) Ni/Mo/Al2O3 (B), and a hydrodesulfurization/-denitrogenation (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. 2.2. Apparatus. A Dionex (Sunnyvale, CA) ASE-300 system was used. A high-pressure vessel having a volume of 34 mL was used as an extraction chamber. The extractor uses varying static times (contact time between the solvent and the matrix, e.g., spent catalyst), different solvents, and elevated temperatures and pressures to increase the efficiency of the extraction process. In parallel, a Soxhlet extraction apparatus (KIMAX 45/50, 24/40) was used to extract the same catalysts at ambient pressure and a temperature according to the boiling points of the solvent (Table 2). 2.3. Materials. All solvents (Table 2), namely, n-heptane (HEP), toluene (TOL), tetrahydrofuran (THF), dichloromethane (DCM), pyridine (PYR), and methanol (MeOH), used in this study were analytical grade with purity of 99+%. Cellulose thimbles from Supelco (cat. no. 64840-U) were used for Soxhlet extraction. (5) Gray, M.; Zhao, Y.; McKnight, C. M. Fuel 2000, 79, 285-294. (6) Van Doorn, J.; Moulijn, J. A. Fuel Process. Technol. 1990, 26, 3951. (7) Kenny, D. V.; Olesik, S. V. J. Chromatogr. Sci. 1998, 36, 59-65, 66-72. (8) Richter, I. E.; Jones, B. A.; Ezzel, J. L.; Porter, N. L. Anal. Chem. 1996, 68, 1033-1039.
10.1021/ef050227l CCC: $33.50 © 2006 American Chemical Society Published on Web 12/23/2005
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Energy & Fuels, Vol. 20, No. 1, 2006 321
Table 1. Characteristics of Catalysts A, B, and C Used in the Present Studya
n-heptane (HEP)
Riedel de Haen cat. no. 32287
98.4
0.012
toluene (TOL)
Scharlau cat. no. Te0079
110.4
0.099
equipped with an automated pneumatic unit for magic-angle spinning (MAS). 2.5.2.1. 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 magicangle spinning (MAS) rate was 13 kHz to minimize overlapping between the main signals (aliphatic and aromatic carbon) and the spinning sidebands. A pulse length of 2.5 µs was used and to be deleted from the end of the line corresponding to the 45° 13C flip angle. The protons were inverse gated decoupled with maximum power (120 W). The recycling delay was 10 s. 2.5.2.2. Liquid-State NMR. 1H NMR 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 as follows: spectral widths of 20 kHz, pulse width of 9 µs. Samples for 1H NMR measurements were prepared by adding 0.5 mL of deuteriochloroform (CDCl3) solvent to 5-10 mg of the extract in a 5-mm tube. Tetramethylsilane (TMS) was used as an internal reference. For 13C measurement, 1.5 mL of CDCl3 was added to 150-250 mg of the soluble extract in a 10-mm tube. The 13C solvent signal at 77.7 ppm was used as internal reference.
tetrahydrofuran (THF)
Scharlau cat. no. Te0225
66.0
0.207
3. Results & Discussion
39.8
0.309
3.1. Extraction. The purpose of the current study was to evaluate the two extraction techniques, Soxhlet and accelerated solvent extraction (ASE), with regard to their efficiency and relevancy for characterization of coke on spent catalysts obtained from industrial atmospheric residue desulfurization (ARDS) units. Compared to the huge number of publications about coke formation on hydrotreating catalysts, there are only a few papers about coke fractionation by graded solubilization of coke or cokelike material using different solvents, such as alkanes,4 aromatics,4,6,10 cyclic ethers,9 halogenated hydrocarbons,6,11-13 or heteroatomic solvents.5 The solvents used in this study differ in their polarity and ability to dissolve hydrocarbons that are constituents of petroleum residues or carbonaceous deposits. At the beginning, the coke on spent catalyst A was fractionated using ASE at 120 °C with different static times to study the time influence on the fraction yield while keeping fixed all other parameters (flush volume: 60%; pressure: 1500 psi, and number of cycles: 2). Although the static times varied from 5 to 60 min, the fraction yields showed only insignificant increases. Table 3 demonstrates the data for experiments performed with highest and lowest static time as well as for comparison the yields from Soxhlet extraction. These data show that ASE with 5-min static time achieved almost the same yields as Soxhlet extraction after 4 h. Thus, all further ASE experiments were performed with 5-min static time to make use of the advantage of shorter extraction durations. To prove that ASE with 5-min static time is able to reach almost the same yields as Soxhlet’s yields, two more types of spent catalysts (B, C) were subjected to ASE and Soxhlet extractions. The ASE conditions were kept as applied for the extraction of catalyst A. The data for catalysts B and C are shown in Table 4. The total extraction yields (sum) were somewhat lower for ASE at 120 °C than for Soxhlet extraction. In detail, especially THF extraction by Soxhlet was much more efficient in comparison with ASE. The results suggest that THF
catalyst characteristics catalyst type bulk density surface area average pore diameter active metal metal content Mo Ni P metal capacity a
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 %
Ref 15. Table 2. Characteristics of Solvents Used
extraction solvent
supplier
dichloromethane (DCM) BDH cat. no. 103406N pyridine (PYR)
Surechem Prod. Ltd. cat. no. P8902
methanol (MeOH)
Riedel de Haen cat. no. 32213
a
boiling point relative (°C) polaritya
115 64.6
0.523 0.762
Values for relative polarity (water ) 1).16
2.4. Extraction Procedures. 2.4.1. ASE. Approximately 10 g of catalyst was packed in a 34-mL extraction cell between two glassfiber filters, Figure 1.9 Each catalyst was sequentially extracted with different solvents (HEP, TOL, THF) and alternatively DCM, PYR, or MeOH. The extract was collected in a receiving bottle. The extraction was performed at different static times (60, 30, 15, and 5 min) at different temperatures (120, 160, 180, and 200 °C). The other operating parameters were pressure (1500 psi), flush volume (60% of cell volume), and extraction cycles (2). 2.4.2. Soxhlet Extraction. About 10 g of catalyst was Soxhletextracted in a conventional Soxhlet apparatus using the same six solvents as used for ASE. The catalyst was placed in a cellulose thimble, which fitted in the Soxhlet apparatus. After no further discoloration of the solvent was observed (∼4 h), the extraction was terminated. The temperature of extraction depended on the boiling point of the extraction solvent (Table 2), where the actual extraction temperature was lower than the boiling point. All extractions (Soxhlet and ASE) were performed in duplicate. Before and after each extraction step, the catalyst was completely dried and weighed. About 1 g of the catalysts was retained for further analyses. 2.4.3. 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 completely dried in a vacuum oven (NU ¨ VE, EV018) at 75 °C and -1.0 bar. The weight of the residue was recorded. 2.5. Analytical Methods. 2.5.1. Elemental Analysis. Elemental analysis (EA) was carried out using an elemental analyzer (i.e., CE Instruments model EA 1110 CHNS). Each sample was analyzed in duplicate. 2.5.2. Nuclear Magnetic Resonance (NMR). The aromaticity of the spent and extracted catalysts as well as the extracts was investigated by solid-state and liquid-state NMR, respectively, using a 300-MHz (7.0463 T) spectrometer from Bruker (AVANCE 300) (9) Dionex Home Page. http://www1.dionex.com/en-us/instruments/ ins7386.html.
(10) Mochida, I.; Zhao, X.; Sakanishi, K.; Yamamoto, S.; Takashima, H.; Uemura, S. Ind. Eng. Chem. Res. 1989, 28, 418-421. (11) Kozhevnikov, I.; Holmes, S.; Siddiqui, M. R. H. Appl. Catal. A: Gen. 2001, 214, 47-58. (12) Holmes, S.; Garforth, M., A. G.; Maunders, B.; Dwyer, J. Appl. Catal. A: Gen. 1997, 151, 355-372. (13) Chu, K. S.; Dong, D.; Hanson, F. V.; Massoth, F. E. Ind. Eng. Chem. Res. 1996, 35, 4012-4019.
322 Energy & Fuels, Vol. 20, No. 1, 2006
Abdullah et al.
Figure 1. (a) Accelerated solvent extractor Dionex-300 schematic diagram. (b) Filling of ASE extraction cell. Table 3. Yield of Solvent-Defined Coke Fractions Obtained from a Spent Catalysts A: Comparison between Soxhlet Extraction and Accelerated Solvent Extraction (ASE) at 120 °C Using Different Static Times solvent-defined coke fractionsa method
HEP-S
TOL-S
THF-S
PYR-S
sum
Soxhlet ASE/60 min ASE/5 min
19.6% 20.1% 20.0%
0.3% 1.2% 1.2%
1.6% 1.2% 0.9%
2.1% 1.6% 1.1%
23.6% 24.1% 23.2%
a HEP-S, TOL-S, THF-S, and PYR-S stand for soluble coke fractions according to the solvent used.
Table 4. Yield of Solvent-Defined Coke Fractions Obtained from Spent Catalysts B and C: Comparison between Soxhlet Extraction and Accelerated Solvent Extraction (ASE) at 120 °C solvent-defined coke fractionsa catalyst method HEP-S TOL- S THF- S PYR-S DCM- S B B
Soxhlet 21.4% 12.6% 11.3% 2.0% ASE 19.6% 8.1% 1.9% 3.8% b -1.8% -4.5% -9.4% +1.8% ∆
C C
Soxhlet 33.5% 1.4% ASE 35.1% 2.3% ∆b +1.6% +0.9%
7.3% 1.2% -6.1%
sum 47.3% 33.3% -14.0%
0.1% 0.1% +0.0%
42.3% 38.7% -3.6%
a HEP-S, TOL-S, THF-S, and DCM-S stand for soluble coke fractions according to the solvent used. b ∆ ) Difference between ASE and Soxhlet yields.
extraction by ASE requires a temperature higher than 120 °C, as explained in a later paragraph. In the finishing extraction step, methanol (MeOH) was tested in addition to pyridine (PYR) and dichloromethane (DCM). Both solvents, MeOH and PYR, were found to be unsuitable under the experimental conditions applied. On one hand, it was impossible to remove PYR from the extracted catalyst and extracts;5 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. These findings suggest that DCM is the most suitable finishing solvent, and it was used in all further experiments. Finally, ASE experiments were performed on catalyst C to optimize the fraction yield by varying the extraction temperature between 120 and 200 °C.14 Table 5 shows the percentage of solvent-defined coke fractions obtained from spent catalyst C, which contained 75 g of coke per 100 g of fresh catalyst. (14) Ali, F. A.; Hauser, A.; Abdullah, H. A.; Al-Adwani, A. Energy Fuel, accepted.
Table 5. Yield of Solvent-Defined Coke Fractions Obtained from a Spent Catalyst C: Comparison between Soxhlet Extraction and Accelerated Solvent Extraction (ASE)a solvent-defined coke fractionsc methodb
HEP-S
TOL-S
THF-S
DCM-S
sum
Soxhlet ASE/200c ASE/180c ASE/160c ASE/120c
33.5% 31.5% 34.0% 31.5% 35.1%
1.4% 0.5% 1.0% 0.9% 2.3%
7.3% 6.5% 7.0% 6.9% 1.2%
0.1% 4.1% 2.3% 0.5% 0.1%
42.3% 42.6% 44.3% 39.8% 38.7%
a Total carbonaceous deposit on catalyst equals 100%. b 200, 180, 160, and 120 stand for the extraction temperature in degree Celsius. c HEP-S, TOL-S, THF-S, and DCM-S stand for soluble coke fractions according to the solvent used.
In ASE, with increasing temperature an increase in the extraction yield is observed (last column in Table 5). Comparing the yields of total extractable matter, it becomes obvious that ASE at about 200 °C is as efficient as Soxhlet extraction. With regard to 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 steps (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 5). THF- and DCM-extraction, however, require high temperature to optimize the fraction yields of THF-S and DCM-S coke, respectively. As Figure 2 demonstrates, the extraction yield of the finishing step (DCM-S coke) can be increased by a factor of 40 by increasing the temperature from 120 to 200 °C. 3.2. Coke Characterization. To investigate the quality of the extracted matter obtained after the ASE and the Soxhlet extraction of catalysts B and C, an elemental analysis was performed for the solvent-related soluble coke fractions. The data are compiled in Tables 6 and 7. In both techniques, regardless of the solvent used, the extracts have a lower C/H ratio than the original coke. Sulfur compounds, it seems, are hard to remove from the coke. Thereby PYR is able to extract an appreciable amount of sulfur compounds; however, the C/N ratios indicate that the PYR-S coke extracts contain a significant (15) Marafi, A.; Al-Bazzaz, H.; Al-Marri, M.; Maruyama, F.; AbsiHalabi, M.; Stanislaus, A. Energy Fuels 2003, 17, 1191-1197. (16) Reichardt, Ch. SolVents and SolVent Effects in Organic Chemistry, 2nd ed.; VCH Verlagsgesellschaft mbH: Weinheim (F.R.G.), 1988.
Coke Extraction of Spent Catalyst
Energy & Fuels, Vol. 20, No. 1, 2006 323
Figure 2. Yield of THF-S and DCM-S coke vs extraction temperature for ASE on a spent catalyst of type B.
Figure 4. catalyst.
Figure 3. catalyst.
13C
NMR spectra of soluble coke fractions from a C-type
Table 6. Elemental Analysis Data of Soluble Extracted Coke from a Spent Catalyst B after Extraction by ASE at 120 °C and Soxhlet ASE solvent
C
cokea
73.2
0.69 142.33
HEP TOL THF DCM PYR
84.91 85.15 84.80 78.82 75.18
0.55 0.57 0.51 0.65 0.74
a
C/H
C/N 495.25 994.00 109.66 278.66 13.29
Soxhlet C/S
C
11.22 73.2 107.30 102.81 418.77 161.68 20.50
C/H
C/N
0.69 142.33
C/S 11.22
84.83 0.58 3297.78 75.38 85.51 0.59 2493.75 95.00 86.45 0.63 504.58 115.33 76.18 0.74 10.84
28.62
Elemental composition of carbonaceous deposit on spent catalyst B.
Table 7. Elemental Analysis Data of Soluble Extracted Coke from a Spent Catalyst C after Extraction by ASE at 120 °C and Soxhlet ASE
Soxhlet
solvent
C
C/H
C/N
C/S
C
C/H
cokea
78.9
0.91
102.28
16.18 78.9
C/N
C/S
0.91 102.28
16.18
HEP TOL THF DCM PYR
85.11 81.54 83.68 90.26 63.49
1.00 992.83 119.44 84.89 0.55 3301.67 0.65 475.42 28.23 84.01 0.75 326.67 0.76 325.50 32.35 86.05 0.61 502.25 0.82 1053.50 602.00 b b b 0.70 5.25 11.44 c c c
102.91 35.56 114.80 b c
a Elemental composition of carbonaceous deposit on spent catalyst C. Traces of extract were collected after DCM extraction. c Soxhlet extraction for this catalyst using pyridine was not conducted.
b
amount of PYR, which cannot be removed as also observed by other authors.5 Further, 13C NMR measurements on the soluble and insoluble coke fractions were performed. The resultant spectra of these fractions obtained either by ASE or by Soxhlet extraction dem-
13C
NMR spectra of insoluble coke fractions on a C-type
onstrate no significant differences in their features. The spectra of the soluble and insoluble fractions of coke on a C-type catalyst are displayed in Figures 3 and 4. After the removal of carryovers from feed, the heptane-insoluble (HEP-IS) coke on catalyst C is mainly aromatic in nature. The aliphatic carbon signal that is observed between 0 and 70 ppm vanishes almost completely in the spectrum of the HEP-IS coke, and the aromaticity of the carbonaceous deposit increases only slightly from the HEP-IS coke (fa ) 0.89) toward the DCM-IS coke (fa ) 0.94). Summary Two extraction techniques, Soxhlet and accelerated solvent extraction (ASE), has been applied to spent catalysts of an industrial ARDS unit to obtain information on the feasibility and efficiency of extraction of carbonaceous deposits on catalysts as well as on characteristics of insoluble coke fractions. ASE was found to be the superior extraction method because it reached the same efficency as Soxhlet after 5 min of extraction compared with 6-12 h by Soxhlet using less solvent. The yields of soluble coke fractions obtained by ASE suggest to begin the extraction at a mild temperature, for example, at 120 °C, and to increase the temperature to the highest possible value (200 °C) before the last two extraction steps. The best ASE conditions were found to be the following: temperature ) 120-200 °C, pressure ) 1500 psi, time ) 5 min, flush volume ) 60%, cycles ) 2, and solvents ) heptane, toluene, tetrahydrofuran, and dichloromethane. Our studies have shown that ASE on spent catalysts is worthwhile because it leads to valuable information about the carbonaceous deposits on spent hydrotreating catalysts. Extraction with heptane or toluene (removal of carryovers from feed or product oil) is a precondition to obtain meaningful data about coke composition. Acknowledgment. The authors give special thanks to Ms. M. Behbehani, Ms. Rab’aa Al-Kandari, and Mr. A. Sultan from the Central Analytical Laboratory (CAL). The authors thank Kuwait National Petroleum Company (KNPC) for supplying spent ARDS catalysts. EF050227L
