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Two-step adsorption process for deep desulfurization of diesel oil
Fuel 84 (2005) 903–910 www.fuelfirst.com
Two-step adsorption process for deep desulfurization of diesel oil Yosuke Sano, Kazuomi Sugahara, Ki-Hyouk Choi, Yozo Korai, Isao Mochida* Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga-Koen, Kasuga-Shi, Fukuoka 816-8580, Japan Received 11 June 2004; received in revised form 30 September 2004; accepted 26 November 2004 Available online 15 December 2004
Abstract An integrated adsorption process for deep desulfurization of diesel fuel was proposed and examined. Conventionally hydrodesulfurized straight run gas oil (HDS-SRGO) having less than 50 ppm sulfur was also adsorptively treated with activated carbon fiber (ACF) to attain the ultra low sulfur gas oil having less than 10 ppm sulfur. The ACF, used in cleaning-up HDS-SRGO, was successively examined in straight run gas oil (SRGO) treatment to enhance its hydrodesulfurization (HDS) reactivity over conventional CoMo catalyst by removing the nitrogen and refractory sulfur species contained in SRGO. Such integrated adsorption–reaction process makes it possible to utilize the maximum adsorption capacity of ACF and achieve ultra deep desulfurization og SRGO. Regeneration of used ACF with a conventional solvent was proved very effective in restoring its adsorption capacity. q 2004 Elsevier Ltd. All rights reserved. Keywords: HDS; Diesel; Activated carbon fiber; Adsorption
1. Introduction Increasing concerns on the air quality have urged the petroleum refining industry to produce cleaner products by removing heteroatoms containing molecules from their major products, diesel and gasoline [1–3]. Although extensive efforts have been made to decrease the sulfur contents of diesel oil [4–8], the regulation on the fuel quality is going to be tightened faster than expected. Furthermore, petroleum refining industry has to compete with non-traditional energy sources, such as natural gas derived oil and hydrogen for the future market [9–12]. Such competition requires the petroleum refining companies to produce cleaner fuel with minimum cost up. Refinery scientists and engineers must solve the above situation. In general, novel catalyst and process concepts have been regarded as the solution for cheaper and cleaner fuel. However, super active catalyst and cutting-edge process technology to meet the future severe regulations on the diesel fuel such as 10, 1 ppmS or 100 ppbS levels will not always be available although numbers of attempts must be also evaluated [13–18]. * Corresponding author. Tel.: C81 92 583 7279; fax: C81 92 583 7798. E-mail address: [email protected] (I. Mochida). 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2004.11.019
Our previous study revealed that activated carbon could adsorb both nitrogen species and refractory sulfur species in gas oil while adsorption capacity for nitrogen species is much larger than that for sulfur species. The adsorptive performance of activated carbons could be improved by oxidative treatment [19–22]. Such observations, in particular, adsorption of refractory sulfur species on activated carbon, indicate the possibility to further remove the sulfur compounds from hydrodesulfurized gas oil by adsorption. Activated carbon is suggested to remove the sulfur species in the current hydrodesulfurized gas oil, containing as much as 100–500 ppmS, to meet the future regulation. Song et al. reported the selective adsorption process for removing sulfur at ambient temperature (SARS) to achieve ultra clean diesel and gasoline [13]. They used 5 wt% metal loaded on silica gel as an adsorbent. Their adsorbents showed rather fast breakthrough over 10 ppmS just after eluting about 5 ml. IRVAD and S-Zorb processes also utilize adsorption to remove the heteroatoms in gasoline and diesel [14–16]. But, their operating temperatures are reported over 240 8C (IRVAD) and 340–410 8C (S-Zorb), respectively. Yang et al. reported that Cu and Ag-exchanged Y-type zeolites were very effective to selectively adsorb thiophene and the used adsorbents could be regenerated by heating them at 350 8C under flowing air [17,18].
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removal of sulfur and nitrogen species from HDS-SRGO over activated carbon fiber (ACF). Their sulfur and nitrogen chromatograms and contents are summarized in Fig. 2 and Table 1. Pitch based activated carbon fiber (ACF), OG-20A, used in this study was supplied by Osaka Gas Co. Its properties are summarized in Table 2. 2.2. Adsorptive treatment of H-SRGO
Fig. 1. The concept of consecutive adsorption process.
In this study, the authors proposed an integrated process for deep desulfurization including the adsorption steps as shown in Fig. 1 where conventionally hydrodesulfurized straight run gas oil (HDS-SRGO) was desulfurized to be less than 10 ppmS over an activated carbon fiber (ACF). The adsorption bed used in the desulfurization of HDS-SRGO could be used again to denitrogenate and desulfurize SRGO as a pre-treatment step of conventional HDS process. Such pre-treatment of SRGO was proved already to enhance markedly the achievable sulfur level. Adsorptive desulfurization concept demonstrated in the present study may be also used to prepare an ultra-clean fuel, which is appropriate for fuel cell application. As shown in Fig. 1, twice use of activated carbon in sequence as an adsorbent for HDS-SRGO and SRGO reduces the frequency of regeneration and oil loss in the adsorption bed. Fully saturated adsorption bed could be regenerated by conventional solvent, such as toluene, as described in previous reports [19–22] and the present study. Activated carbon fiber (ACF) was selected in this study as an adsorbent because it showed rather low pressure drop and high performance among the activated carbon materials examined in previous reports [19–22].
ACF (0.3 g) dried at 110 8C under vacuum for 2 h was packed into the stainless steel tube of 50 mm length and 6 mm diameter. Gas oil was fed into the tube by an HPLC pump at the rate of 0.1 ml/min and at the pressure of about 20 psi. The adsorption temperature was adjusted to 0–70 8C by a water bath. The eluted oil through ACF was sampled as much as 0.05 ml at every 60 min and their sulfur and nitrogen contents were analyzed by GC (HP 6890P) coupled with an AED (Atomic Emission Detector, G2350A [23–25]). Ten weight percent of toluene, decane, decaline, tetralin and 10–50 wt% of 1-metylnaphthalene were added into HDS-SRGO to investigate the effects of added aromatic, paraffinic and naphthenic molecules on the adsorption of sulfur species over the ACF. 2.3. Consecutive adsorption treatment of H-SRGO and SRGO The consecutive adsorption experiment was performed as follows † HDS- or AD-HDS-SRGO was fed into the ACF under the conditions described above, until the sulfur content in eluted oil reached 10 ppmS. † Feed was changed to SRGO in order to remove its nitrogen and sulfur species. 2.4. Hydrodesulfurization of adsorptively treated gas oils
2. Experimental
Eluted oil from 80 ml SRGO over 1 g of ACF was collected and hydrodesulfurized by using an autoclave-type reactor of 100 ml internal volume. Catalyst used in the HDS was a commercially available CoMo/SiO2–Al2O3, which was pre-sulfided by 5% H2S/H2 at 360 8C for 2 h. Ten gram oil, 1 g catalyst, and 50 kg/cm2 hydrogen gas were charged into the reactor at room temperature. Reaction temperature and time was 340 8C and 2 h, respectively. Reactor pressure was about 70 kg/cm2 during reaction.
2.1. Feed gas oils and activated carbon fiber
2.5. Regeneration of the used activated carbon fiber
SRGO and its conventionally hydrodesulfurized straight run gas oil (HDS-SRGO) were supplied by a Japan refinery. Adsorptively desulfurized HDS-SRGO (AD-HDS-SRGO) was prepared by the present authors through the adsorptive
Used ACF, saturated with sulfur and nitrogen species after consecutive adsorption experiment was regenerated by aromatic solvents, such as toluene, 1-methyl-naphthalene and tetralin. Used ACF was dipped into one of the solvents
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Fig. 2. Sulfur and nitrogen chromatograms of (A) SRGO, (B) HDS-SRGO and (C) AD-HDS-SRGO. (1) 4-Methyldibenzothiophene, (2) 4,6dimethyldibenzothiophene, (3) 2,4,6-trimethyldibenzothiophene, (4) 1-methylcarbazole, (5) 1,8-dimethylcarbazole (6) 1,4,8-trimethylcarbazole.
(10–50 ml) for 2 h at 70 8C under ultrasonic radiation. Then, ACF was filtered and dried at 120 8C under vacuum. The solvent recovered after extraction was also analyzed by GCAED.
3. Result 3.1. Feed characterization SRGO feed contained benzothiophenes (BTs) and dibenzothiophenes (DBTs) as shown in Fig. 2. Its hydrodesulfurized oil (HDS-SRGO) showed the dominant presence of 4-methyldibenzothiophene (4-MDBT), 4,6dimethyldibenzothiophene (4,6-DMDBT) and 2,4,6-trimethyldibenzothiophene (2,4,6-TMDBT), which have been regarded as the most refractory sulfur species. The most Table 1 Sulfur and nitrogen content in gas oils
SRGO HDS-SRGO AD-HDS-SRGO a
Below 1 ppm.
abundant sulfur molecule in HDS-SRGO was 4,6-dimethyldibenzothiophene (50 ppmS). AD-HDS-SRGO, which was adsorptively treated gas oil of HDS-SRGO, contained similar sulfur species with those of HDS-SRGO. However, the contents of those species were varied; 4-MDBT was vanished. Most of nitrogen species found in SRGO were carbazoles (Cz) as indicated in Fig. 2. Refractory nitrogen species, such as 1-methylcarbazole (1-MCz), 1,8-dimethylcarbazole (1,8-DMCz) and 1,4,8-trimethylcarbazole (1,4,8-TMCz) were found in HDS-SRGO. Adsorptive treatment of HDSSRGO removed almost all of nitrogen species as shown in Fig. 2(C). 3.2. Adsorptive removal of sulfur species in HDS-SRGOs Fig. 3 shows the sulfur chromatograms of AD-HDSSRGO (50 ppmS) and its eluted oils. Eluted oils after 20 ml feeding over 1 g of ACF showed only two peaks of Table 2 The property of OG-20A
Sulfur (ppm)
Nitrogen (ppm)
11780 300 50
260 30 0a
Elemental analysis (%)
Surface area (m2/g)
H
C
N
O
Ash
0.73
94.64
0.26
4.05
0.32
1900
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Fig. 3. Sulfur chromatograms of (A) feed oil (AD-HDS-SRGO) (B), (C) eluted oils after (B) 40 ml and (C) 20 ml oil were fed. The numbers on each chromatograms indicate the total sulfur content. Adsorbent: 1 g of OG-20A. Adsorption temperature: 30 8C. (1) 4-Methyldibenzothiophene, (2) 4,6dimethyldibenzothiophene, (3) 4-ethyl, 6-methyldibenzothiophene, (4) 4,6diethyldibenzothiophene.
mono-methyl DBTs and one peak of dimethyl-mono-ethyl DBT. Ninety seven and 88% of sulfur species contained in AD-HDS-SRGO were removed after 20 and 40 ml feeding, respectively. Fig. 4 shows the sulfur breakthrough profiles of HDSand AD-HDS-SRGO over the ACF at 30 8C. Seventy mililiter of AD-HDS-SRGO could be treated per 1 g ACF until the sulfur content in eluted oil exceeded 10 ppmS whereas only 12 ml of HDS-SRGO could be treated per 1 g ACF to satisfy the less than ‘10 ppmS’. Sulfur contents in the accumulated elutants of 10, 40 and 80 ml from ADHDS-SRGO per 1 g ACF were 0.5, 2.7 and 8.2 ppmS, respectively.
Fig. 4. Sulfur breakthrough profiles of (A) HDS-SRGO (300 ppmS) and (B) AD-HDS-SRGO (50 ppmS). Adsorption temperature: 30 8C.
Fig. 5. Temperature effects on removed amount of sulfur species. Adsorbent: 0.3 g of OG-20A. Feed: 36 ml of HDS-SRGO (300 ppmS).
3.3. The effects of adsorption temperature on the adsorption capacity for sulfur species Fig. 5 shows the adsorptively removed amount of sulfur species from HDS-SRGO at several temperatures. The largest adsorption capacity was attained at 30 8C while higher and lower temperature than 30 8C decreased the adsorption capacity. 3.4. Effects of aromatic additives Fig. 6 shows the sulfur chromatograms of elutants from HDS-SRGO and diluted one with 1-methylnaphthalene (50 wt%). Although diluted HDS-SRGO with 1-methyl naphthalene carried 50% sulfur content of HDS-SRGO, its eluted oil contained higher sulfur content than that of HDS-SRGO after the same adsorption treatment, indicating the strong inhibition of 1-methylnaphthalene against
Fig. 6. Sulfur chromatogram of adsorptive treated H1-SRGOs. (A) Asreceived HDS-SRGO (B) 50 wt% 1-methyl naphthalene added HDSSRGO. Adsorbent: 0.3 g of OG-20A. Adsorption temperature: 30 8C. Feed: 36 ml of HDS-SRGO and diluted HDS-SRGO.
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Table 3 The effects of coexists in HDS-SRGO on adsorption capacity of sulfur species Feed
Removed amount of sulfur species (mg-sulfur)
HDS-SRGO HDS-SRGOC10%-decane HDS-SRGOC10%-decalin HDS-SRGOC10%-1-metyl naphthalene HDS-SRGOC30%-1-metyl naphthalene HDS-SRGOC50%-1-metyl naphthalene HDS-SRGOC30%-toluene HDS-SRGOC30%-tetralin
3.13 2.67 2.92 1.82 0.42 0.23 1.88 1.46
Adsorbent: 0.3 g of OG-20A; adsorption temperature: 30 8C; oil: 30 ml of H1-SRGO with additives.
the adsorption of sulfur species over the ACF. The distribution of sulfur species on the chromatogram was almost the same, in spite of inhibition by 1-methylnaphthalene. Table 3 summarizes the removed amount of sulfur species over ACF from non-diluted and diluted HDS-SRGO feeds. The inhibition of 1-methylnaphthalene was most severe among the additives. Fifty percent dilution by 1methylnaphthalene reduced the adsorption capacity by an order of magnitude. On the other hand, paraffin (decane) and naphthene (decaline) did hardly inhibit the adsorption of sulfur species on ACF.
Fig. 8. Sulfur chromatograms of (A) SRGO, (B) and (C) eluted oils through OG-20A, which was used to treat (B) HDS-SRGO (300 ppmS) and (C) ADHDS-SRGO (50 ppmS). Adsorbent: 0.3 g of sulfur saturated OG-20A. Adsorption temperature: 30 8C. Feed: 36 ml of SRGO (11780 ppmS and 260 ppmN).
Fig. 7 shows the nitrogen breakthrough profiles of SRGO over the virgin and used ACF. All of ACFs showed almost the same profiles except for one point at 40 ml elution over the ACF which was used to treat 100 ml of HDS-SRGO prior to SRGO feeding. Such high nitrogen content observed
in the early elution suggested that the removal of nitrogen species in SRGO was retarded by pre-adsorbed nitrogen species. However, ACF which was used to treat 20 ml of HDS-SRGO or 100 ml of AD-HDS-SRGO per 1 g ACF did not show such a spike in the nitrogen breakthrough profile. Fig. 8 shows the sulfur chromatograms of feed SRGO and adsorptively denitrogenated SRGOs. A little amount of sulfur species in SRGO, especially those in high boiling temperature range, were removed over used ACF which had been used to give 80 ml of less than 10 ppmS gas oil from AD-HDS-SRGO. However, the other used ACF, which was saturated with sulfur species in HDS-SRGO, could not adsorb the sulfur species in SRGO furthermore. Fig. 9 shows the sulfur chromatograms of HDS products of feed SRGO and SRGO adsorptively treated over used
Fig. 7. Nitrogen breakthrough profiles from SRGO over virgin and sulfursaturated OG-20A*. Adsorption temperature: 30 8C. (A) Virgin OG-20A. (B) Used OG-20A (100 ml of H2-SRGO treated). (C) Used OG-20A (100 ml of H1-SRGO treated). (D) Used OG-20A (20 ml of H1-SRGO treated). *Sulfur saturated activated carbon: OG-20A which was used as an adsorbent to remove sulfur species in HDS or AD-HDS-SRGO.
Fig. 9. Sulfur chromatograms of HDS products from (A) SRGO (11780 ppmS, 260 ppmN) and (B) adsorptively treated SRGO (11400 ppmS, 40 ppmN). (1) 4-Methyldibenzothiophene, (2) 4,6dimethyldibenzothiophene, (3) 4-ethyl, 6-methyldibenzothiophene, (4) 1,4,6-trimethyldibenzothiophene, (5) 4,6-diethyldibenzothiophene.
3.5. Adsorptive removal of nitrogen species in SRGO over ACF
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ACF which had been used to treat 80 ml of AD-HDS-SRGO prior to SRGO-treatment. Adsorptively treated SRGO (sulfur content: 11400 ppmS, nitrogen content: 40 ppmN) left only 40 ppmS while as supplied SRGO (sulfur content: 11780 ppmS, nitrogen content: 260 ppmN) did as much as 193 ppmS sulfur after HDS reaction over a commercially available CoMo catalyst even under the same reaction conditions. 3.6. Regeneration of activated carbon fiber After 80 ml AD-HDS-SRGO and 80 ml SRGO were treated, adsorbed sulfur and nitrogen species over used OG-20A (ACF) were extracted by various solvent. Sulfur and nitrogen content in the solvent was plotted in Fig. 10. 1-Methylnaphthalene extracted the largest amount of sulfur and nitrogen species from the ACF among the solvents. Toluene and tetralin could also extract sulfur and nitrogen species from OG-20A (ACF). Fig. 11 shows the removal amount of sulfur species in HDS-SRGO and nitrogen species in SRGO over the regenerated ACF. Regenerated
Fig. 11. Removal amount of sulfur species in HDS-SRGO and nitrogen in SRGO over virgin and regenerated ACF. Adsorbent: 0.3 g of OG-20A. Adsorption temperature: 30 8C. Feed: 30 ml of HDS-SRGO and SRGO.
ACF showed the same adsorption capacity for nitrogen in SRGO and sulfur species in HDS-SRGO.
4. Discussion The achievements of the present paper are summarized as followed. 1. Conventionally hydrodesulfurized gas oil carrying 50– 300 ppmS could be desulfurized to be less than 10 ppmS by adsorption over ACF at room temperature. 2. ACF bed could be used twice. ACF bed, which was used to adsorptively treat conventionally hydrotreated SRGO to be less than 10 ppmS, could further remove sulfur and nitrogen species of SRGO. Adsorptively treated SRGO showed very improved reactivity toward hydrodesulfurization. 3. Used ACF could be successfully regenerated by the aromatic solvent and extracted nitrogen and sulfur species from ACF with solvent can be recovered by distillation.
Fig. 10. Sulfur and nitrogen content of eluted solvents through exhausted ACF. Adsorbent: 0.3 g of OG-20A. Solvent-treating temperature: 70 8C. Solvent: toluene, 1-methyl-naphthalene and tetralin.
It is very critical task to assure the sulfur content of diesel oil to be less than regulated value, that is 10 ppmS in Euro IV. In general, refineries control the reaction temperature, space velocity and hydrogen/feed oil ratio to maintain the product specifications. However, the tolerance range in operating condition must be very narrow if the product specifications are ultimately limited. Hence, some kinds of back up facility should be installed to protect the run away of product quality, which is frequently encountered in the practical operation. Additional reactor, blending facility may be a solution for such emergency situation. The present authors suggest the adsorption bed as the back up facility to desulfurize the diesel having off-specification.
Y. Sano et al. / Fuel 84 (2005) 903–910
Adsorptive removal of sulfur species from the currently available hydrotreated gas oil (HDS-SRGO) of 300 ppmS to get the ultra low sulfur diesel described in the present paper may be impractical due to very short operation time to achieve 10 ppmS as shown in Fig. 4. However, deeply hydrotreated gas oil of 50 ppmS, which could be easily attained with the conventional HDS reaction when nitrogen species in SRGO are removed prior to HDS, showed much longer breakthrough time to reach 10 ppmS. Thus, the present process requires hydrodesulfurization of SRGO to be less than 50 ppmS in order to obtain the acceptable amount of adsorptively treated oil. Furthermore, early eluted gas oil can be used for fuel cell because it contained less than 1 ppmS. HDS of SRGO to be less than 50 ppmS could be achieved more easily by the pre-treatment to remove nitrogen species over ACF as described in our previous papers [19–22] and Fig. 7. Thus, ACF bed can be used consecutively for the adsorbent for the both of pre- and post-treatment steps. A particular temperature of 30 8C appears to absorb the largest amounts of sulfur species in HDS-SRGO over OG20A. Some of sulfur species, which are weakly adsorbed over ACF at 30 8C, appears to be desorbed at a higher temperature. On the other hand, it is not recommended to the present process to be operated at lower temperature than 30 8C due to the low fluidity of the gas oil at low temperature. It was found that addition of hydrocarbons having more than 10 carbons decreased the adsorption capacity of ACF. Especially, 1-methylnaphthalene of even 10% decreased the adsorption amount of sulfur to be a half. Such results indicate the limitation of the present process to treat the highly aromatic gas oil such as light cycle oil. Anti-solvent such as pressurized propane is worthwhile for trying to relieve the inhibition by aromatics, but application may be expensive due to high pressure for liquefying propane. ACF bed saturated with sulfur and nitrogen species could be successfully regenerated by conventional aromatic solvents as shown in Fig. 11. Naphthene can be also recommended as the solvent for regeneration of ACF. Hence, the life time of adsorbents, which is a very important factor in the practical operation, is believed to be long enough to minimize the operation cost. Repeated use of ACF does not lower the performance but reduce the cost of adsorbent. When the pre- and posttreatment steps have each adsorption bed, four adsorption beds must be installed (two for adsorption, two for regeneration) for continuous operation whereas the present integrated process needs three (two for two adsorption, one for regeneration) as shown in Fig. 12. This leads to the reduction of initial investment. The used ACF, which treated HDS-SRGO to keep the elutant less than 10 ppmS, showed almost the same breakthrough profiles of nitrogen species in SRGO as the virgin one. Repeated use of ACF reduces not only oil loss but also the frequency of
909
Fig. 12. Comparing the one bed system with two bed system.
regeneration, which indicates the good economy of present process. At the regeneration step, some amount of oil must be remained in the bed not to be lost. Loss of HDS-SRGO was reduced at its adsorptive removal to be included into SRGO, contributed the lower sulfur level of SRGO after HDS.
5. Conclusions A novel integrated process including adsorption system for pre- and post-treatment of gas oil was proved to achieve ultra deep desulfurization by using the OG-20A (ACF) as an adsorbent. Consecutive use of ACF for HDS-SRGO and SRGO reduced the number of adsorption bed, loss of feed oil and severity of HDS by removing the nitrogen and refractory sulfur species in SRGO. HDS-SRGO obtained by HDS is recommended to have less than 50 ppmS to be processed in the present adsorption system.
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Segawa K, Takahashi K, Satoh S. Catal Today 2000;63:123. Takatsuka T, Inoue S, Wada Y. Catal Today 1997;39:69. MacLean HL, Lave LB. Prog Energy Combust Sci 2003;29:69. Song C. Catal Today 2003;86:211. Mochida I, Sakanishi K, Ma X, Nagao S, Isoda T. Catal Today 1996; 29:185. Choi K-H, Kunisada N, Korai Y, Mochida I, Nakano K. Catal Today 2003;86:277. Kunisada N, Choi K-H, Korai Y, Mochida I. Appl Catal A 2004; 260:185. Choi K-H, Korai Y, Mochida I. Appl Catal A 2004;260:229. Ale Ebrahim H, Jamshidi E. Energy Convers Manage 2004;45:345. Sydney T, Richard AD. Energy 2003;28:1461. Daniel GL, Kyle T, Dylan M. J Power Sources 2003;117:84. Hirsch D, Epstein M, Steinfeld A. Int J Hydrogen Energy 2001;26: 1023.
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Y. Sano et al. / Fuel 84 (2005) 903–910 Song C, Ma X. Appl Catal B 2003;41:207. Khare GP. US patent 6,346,190; 2002. Salem AS, Hamid HS. Chem Eng Technol 1997;20:342. Irvine RL. US Patent 5,730,860; 1998. Hernandez-Maldonado AJ, Yang RT. Ind Eng Chem Res 2003;42:123. Takahashi A, Yang FH, Yang RT. Ind Eng Chem Res 2003;41:2487. Sano Y, Choi K-H, Korai Y, Mochida I. Appl Catal B 2003;49:219. Sano Y, Choi K-H, Korai Y, Mochida I. Energy Fuels 2004;18: 644.
[21] Sano Y, Choi K-H, Korai Y, Mochida I. Prepr Pap Am Chem Soc, Div Fuel Chem 2003;48(1):138. [22] Sano Y, Choi K-H, Korai Y, Mochida I. Prepr Pap Am Chem Soc, Div Fuel Chem 2003;48(2):658. [23] Sumbogo SD, Yang H, Choi K-H, Korai Y, Mochida I. Appl Catal A 2003;252:331. [24] Shin S, Sakanishi K, Mochida I, Grudoski DA, Shinn JH. Energy Fuels 2000;14:539. [25] Miki Y, Sugimoto Y, Tanaka M, Wu Z. J Jpn Pet Inst 2002;45:117.
Two-step adsorption process for deep desulfurization of diesel oil Yosuke Sano, Kazuomi Sugahara, Ki-Hyouk Choi, Yozo Korai, Isao Mochida* Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga-Koen, Kasuga-Shi, Fukuoka 816-8580, Japan Received 11 June 2004; received in revised form 30 September 2004; accepted 26 November 2004 Available online 15 December 2004
Abstract An integrated adsorption process for deep desulfurization of diesel fuel was proposed and examined. Conventionally hydrodesulfurized straight run gas oil (HDS-SRGO) having less than 50 ppm sulfur was also adsorptively treated with activated carbon fiber (ACF) to attain the ultra low sulfur gas oil having less than 10 ppm sulfur. The ACF, used in cleaning-up HDS-SRGO, was successively examined in straight run gas oil (SRGO) treatment to enhance its hydrodesulfurization (HDS) reactivity over conventional CoMo catalyst by removing the nitrogen and refractory sulfur species contained in SRGO. Such integrated adsorption–reaction process makes it possible to utilize the maximum adsorption capacity of ACF and achieve ultra deep desulfurization og SRGO. Regeneration of used ACF with a conventional solvent was proved very effective in restoring its adsorption capacity. q 2004 Elsevier Ltd. All rights reserved. Keywords: HDS; Diesel; Activated carbon fiber; Adsorption
1. Introduction Increasing concerns on the air quality have urged the petroleum refining industry to produce cleaner products by removing heteroatoms containing molecules from their major products, diesel and gasoline [1–3]. Although extensive efforts have been made to decrease the sulfur contents of diesel oil [4–8], the regulation on the fuel quality is going to be tightened faster than expected. Furthermore, petroleum refining industry has to compete with non-traditional energy sources, such as natural gas derived oil and hydrogen for the future market [9–12]. Such competition requires the petroleum refining companies to produce cleaner fuel with minimum cost up. Refinery scientists and engineers must solve the above situation. In general, novel catalyst and process concepts have been regarded as the solution for cheaper and cleaner fuel. However, super active catalyst and cutting-edge process technology to meet the future severe regulations on the diesel fuel such as 10, 1 ppmS or 100 ppbS levels will not always be available although numbers of attempts must be also evaluated [13–18]. * Corresponding author. Tel.: C81 92 583 7279; fax: C81 92 583 7798. E-mail address: [email protected] (I. Mochida). 0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2004.11.019
Our previous study revealed that activated carbon could adsorb both nitrogen species and refractory sulfur species in gas oil while adsorption capacity for nitrogen species is much larger than that for sulfur species. The adsorptive performance of activated carbons could be improved by oxidative treatment [19–22]. Such observations, in particular, adsorption of refractory sulfur species on activated carbon, indicate the possibility to further remove the sulfur compounds from hydrodesulfurized gas oil by adsorption. Activated carbon is suggested to remove the sulfur species in the current hydrodesulfurized gas oil, containing as much as 100–500 ppmS, to meet the future regulation. Song et al. reported the selective adsorption process for removing sulfur at ambient temperature (SARS) to achieve ultra clean diesel and gasoline [13]. They used 5 wt% metal loaded on silica gel as an adsorbent. Their adsorbents showed rather fast breakthrough over 10 ppmS just after eluting about 5 ml. IRVAD and S-Zorb processes also utilize adsorption to remove the heteroatoms in gasoline and diesel [14–16]. But, their operating temperatures are reported over 240 8C (IRVAD) and 340–410 8C (S-Zorb), respectively. Yang et al. reported that Cu and Ag-exchanged Y-type zeolites were very effective to selectively adsorb thiophene and the used adsorbents could be regenerated by heating them at 350 8C under flowing air [17,18].
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Y. Sano et al. / Fuel 84 (2005) 903–910
removal of sulfur and nitrogen species from HDS-SRGO over activated carbon fiber (ACF). Their sulfur and nitrogen chromatograms and contents are summarized in Fig. 2 and Table 1. Pitch based activated carbon fiber (ACF), OG-20A, used in this study was supplied by Osaka Gas Co. Its properties are summarized in Table 2. 2.2. Adsorptive treatment of H-SRGO
Fig. 1. The concept of consecutive adsorption process.
In this study, the authors proposed an integrated process for deep desulfurization including the adsorption steps as shown in Fig. 1 where conventionally hydrodesulfurized straight run gas oil (HDS-SRGO) was desulfurized to be less than 10 ppmS over an activated carbon fiber (ACF). The adsorption bed used in the desulfurization of HDS-SRGO could be used again to denitrogenate and desulfurize SRGO as a pre-treatment step of conventional HDS process. Such pre-treatment of SRGO was proved already to enhance markedly the achievable sulfur level. Adsorptive desulfurization concept demonstrated in the present study may be also used to prepare an ultra-clean fuel, which is appropriate for fuel cell application. As shown in Fig. 1, twice use of activated carbon in sequence as an adsorbent for HDS-SRGO and SRGO reduces the frequency of regeneration and oil loss in the adsorption bed. Fully saturated adsorption bed could be regenerated by conventional solvent, such as toluene, as described in previous reports [19–22] and the present study. Activated carbon fiber (ACF) was selected in this study as an adsorbent because it showed rather low pressure drop and high performance among the activated carbon materials examined in previous reports [19–22].
ACF (0.3 g) dried at 110 8C under vacuum for 2 h was packed into the stainless steel tube of 50 mm length and 6 mm diameter. Gas oil was fed into the tube by an HPLC pump at the rate of 0.1 ml/min and at the pressure of about 20 psi. The adsorption temperature was adjusted to 0–70 8C by a water bath. The eluted oil through ACF was sampled as much as 0.05 ml at every 60 min and their sulfur and nitrogen contents were analyzed by GC (HP 6890P) coupled with an AED (Atomic Emission Detector, G2350A [23–25]). Ten weight percent of toluene, decane, decaline, tetralin and 10–50 wt% of 1-metylnaphthalene were added into HDS-SRGO to investigate the effects of added aromatic, paraffinic and naphthenic molecules on the adsorption of sulfur species over the ACF. 2.3. Consecutive adsorption treatment of H-SRGO and SRGO The consecutive adsorption experiment was performed as follows † HDS- or AD-HDS-SRGO was fed into the ACF under the conditions described above, until the sulfur content in eluted oil reached 10 ppmS. † Feed was changed to SRGO in order to remove its nitrogen and sulfur species. 2.4. Hydrodesulfurization of adsorptively treated gas oils
2. Experimental
Eluted oil from 80 ml SRGO over 1 g of ACF was collected and hydrodesulfurized by using an autoclave-type reactor of 100 ml internal volume. Catalyst used in the HDS was a commercially available CoMo/SiO2–Al2O3, which was pre-sulfided by 5% H2S/H2 at 360 8C for 2 h. Ten gram oil, 1 g catalyst, and 50 kg/cm2 hydrogen gas were charged into the reactor at room temperature. Reaction temperature and time was 340 8C and 2 h, respectively. Reactor pressure was about 70 kg/cm2 during reaction.
2.1. Feed gas oils and activated carbon fiber
2.5. Regeneration of the used activated carbon fiber
SRGO and its conventionally hydrodesulfurized straight run gas oil (HDS-SRGO) were supplied by a Japan refinery. Adsorptively desulfurized HDS-SRGO (AD-HDS-SRGO) was prepared by the present authors through the adsorptive
Used ACF, saturated with sulfur and nitrogen species after consecutive adsorption experiment was regenerated by aromatic solvents, such as toluene, 1-methyl-naphthalene and tetralin. Used ACF was dipped into one of the solvents
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Fig. 2. Sulfur and nitrogen chromatograms of (A) SRGO, (B) HDS-SRGO and (C) AD-HDS-SRGO. (1) 4-Methyldibenzothiophene, (2) 4,6dimethyldibenzothiophene, (3) 2,4,6-trimethyldibenzothiophene, (4) 1-methylcarbazole, (5) 1,8-dimethylcarbazole (6) 1,4,8-trimethylcarbazole.
(10–50 ml) for 2 h at 70 8C under ultrasonic radiation. Then, ACF was filtered and dried at 120 8C under vacuum. The solvent recovered after extraction was also analyzed by GCAED.
3. Result 3.1. Feed characterization SRGO feed contained benzothiophenes (BTs) and dibenzothiophenes (DBTs) as shown in Fig. 2. Its hydrodesulfurized oil (HDS-SRGO) showed the dominant presence of 4-methyldibenzothiophene (4-MDBT), 4,6dimethyldibenzothiophene (4,6-DMDBT) and 2,4,6-trimethyldibenzothiophene (2,4,6-TMDBT), which have been regarded as the most refractory sulfur species. The most Table 1 Sulfur and nitrogen content in gas oils
SRGO HDS-SRGO AD-HDS-SRGO a
Below 1 ppm.
abundant sulfur molecule in HDS-SRGO was 4,6-dimethyldibenzothiophene (50 ppmS). AD-HDS-SRGO, which was adsorptively treated gas oil of HDS-SRGO, contained similar sulfur species with those of HDS-SRGO. However, the contents of those species were varied; 4-MDBT was vanished. Most of nitrogen species found in SRGO were carbazoles (Cz) as indicated in Fig. 2. Refractory nitrogen species, such as 1-methylcarbazole (1-MCz), 1,8-dimethylcarbazole (1,8-DMCz) and 1,4,8-trimethylcarbazole (1,4,8-TMCz) were found in HDS-SRGO. Adsorptive treatment of HDSSRGO removed almost all of nitrogen species as shown in Fig. 2(C). 3.2. Adsorptive removal of sulfur species in HDS-SRGOs Fig. 3 shows the sulfur chromatograms of AD-HDSSRGO (50 ppmS) and its eluted oils. Eluted oils after 20 ml feeding over 1 g of ACF showed only two peaks of Table 2 The property of OG-20A
Sulfur (ppm)
Nitrogen (ppm)
11780 300 50
260 30 0a
Elemental analysis (%)
Surface area (m2/g)
H
C
N
O
Ash
0.73
94.64
0.26
4.05
0.32
1900
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Fig. 3. Sulfur chromatograms of (A) feed oil (AD-HDS-SRGO) (B), (C) eluted oils after (B) 40 ml and (C) 20 ml oil were fed. The numbers on each chromatograms indicate the total sulfur content. Adsorbent: 1 g of OG-20A. Adsorption temperature: 30 8C. (1) 4-Methyldibenzothiophene, (2) 4,6dimethyldibenzothiophene, (3) 4-ethyl, 6-methyldibenzothiophene, (4) 4,6diethyldibenzothiophene.
mono-methyl DBTs and one peak of dimethyl-mono-ethyl DBT. Ninety seven and 88% of sulfur species contained in AD-HDS-SRGO were removed after 20 and 40 ml feeding, respectively. Fig. 4 shows the sulfur breakthrough profiles of HDSand AD-HDS-SRGO over the ACF at 30 8C. Seventy mililiter of AD-HDS-SRGO could be treated per 1 g ACF until the sulfur content in eluted oil exceeded 10 ppmS whereas only 12 ml of HDS-SRGO could be treated per 1 g ACF to satisfy the less than ‘10 ppmS’. Sulfur contents in the accumulated elutants of 10, 40 and 80 ml from ADHDS-SRGO per 1 g ACF were 0.5, 2.7 and 8.2 ppmS, respectively.
Fig. 4. Sulfur breakthrough profiles of (A) HDS-SRGO (300 ppmS) and (B) AD-HDS-SRGO (50 ppmS). Adsorption temperature: 30 8C.
Fig. 5. Temperature effects on removed amount of sulfur species. Adsorbent: 0.3 g of OG-20A. Feed: 36 ml of HDS-SRGO (300 ppmS).
3.3. The effects of adsorption temperature on the adsorption capacity for sulfur species Fig. 5 shows the adsorptively removed amount of sulfur species from HDS-SRGO at several temperatures. The largest adsorption capacity was attained at 30 8C while higher and lower temperature than 30 8C decreased the adsorption capacity. 3.4. Effects of aromatic additives Fig. 6 shows the sulfur chromatograms of elutants from HDS-SRGO and diluted one with 1-methylnaphthalene (50 wt%). Although diluted HDS-SRGO with 1-methyl naphthalene carried 50% sulfur content of HDS-SRGO, its eluted oil contained higher sulfur content than that of HDS-SRGO after the same adsorption treatment, indicating the strong inhibition of 1-methylnaphthalene against
Fig. 6. Sulfur chromatogram of adsorptive treated H1-SRGOs. (A) Asreceived HDS-SRGO (B) 50 wt% 1-methyl naphthalene added HDSSRGO. Adsorbent: 0.3 g of OG-20A. Adsorption temperature: 30 8C. Feed: 36 ml of HDS-SRGO and diluted HDS-SRGO.
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Table 3 The effects of coexists in HDS-SRGO on adsorption capacity of sulfur species Feed
Removed amount of sulfur species (mg-sulfur)
HDS-SRGO HDS-SRGOC10%-decane HDS-SRGOC10%-decalin HDS-SRGOC10%-1-metyl naphthalene HDS-SRGOC30%-1-metyl naphthalene HDS-SRGOC50%-1-metyl naphthalene HDS-SRGOC30%-toluene HDS-SRGOC30%-tetralin
3.13 2.67 2.92 1.82 0.42 0.23 1.88 1.46
Adsorbent: 0.3 g of OG-20A; adsorption temperature: 30 8C; oil: 30 ml of H1-SRGO with additives.
the adsorption of sulfur species over the ACF. The distribution of sulfur species on the chromatogram was almost the same, in spite of inhibition by 1-methylnaphthalene. Table 3 summarizes the removed amount of sulfur species over ACF from non-diluted and diluted HDS-SRGO feeds. The inhibition of 1-methylnaphthalene was most severe among the additives. Fifty percent dilution by 1methylnaphthalene reduced the adsorption capacity by an order of magnitude. On the other hand, paraffin (decane) and naphthene (decaline) did hardly inhibit the adsorption of sulfur species on ACF.
Fig. 8. Sulfur chromatograms of (A) SRGO, (B) and (C) eluted oils through OG-20A, which was used to treat (B) HDS-SRGO (300 ppmS) and (C) ADHDS-SRGO (50 ppmS). Adsorbent: 0.3 g of sulfur saturated OG-20A. Adsorption temperature: 30 8C. Feed: 36 ml of SRGO (11780 ppmS and 260 ppmN).
Fig. 7 shows the nitrogen breakthrough profiles of SRGO over the virgin and used ACF. All of ACFs showed almost the same profiles except for one point at 40 ml elution over the ACF which was used to treat 100 ml of HDS-SRGO prior to SRGO feeding. Such high nitrogen content observed
in the early elution suggested that the removal of nitrogen species in SRGO was retarded by pre-adsorbed nitrogen species. However, ACF which was used to treat 20 ml of HDS-SRGO or 100 ml of AD-HDS-SRGO per 1 g ACF did not show such a spike in the nitrogen breakthrough profile. Fig. 8 shows the sulfur chromatograms of feed SRGO and adsorptively denitrogenated SRGOs. A little amount of sulfur species in SRGO, especially those in high boiling temperature range, were removed over used ACF which had been used to give 80 ml of less than 10 ppmS gas oil from AD-HDS-SRGO. However, the other used ACF, which was saturated with sulfur species in HDS-SRGO, could not adsorb the sulfur species in SRGO furthermore. Fig. 9 shows the sulfur chromatograms of HDS products of feed SRGO and SRGO adsorptively treated over used
Fig. 7. Nitrogen breakthrough profiles from SRGO over virgin and sulfursaturated OG-20A*. Adsorption temperature: 30 8C. (A) Virgin OG-20A. (B) Used OG-20A (100 ml of H2-SRGO treated). (C) Used OG-20A (100 ml of H1-SRGO treated). (D) Used OG-20A (20 ml of H1-SRGO treated). *Sulfur saturated activated carbon: OG-20A which was used as an adsorbent to remove sulfur species in HDS or AD-HDS-SRGO.
Fig. 9. Sulfur chromatograms of HDS products from (A) SRGO (11780 ppmS, 260 ppmN) and (B) adsorptively treated SRGO (11400 ppmS, 40 ppmN). (1) 4-Methyldibenzothiophene, (2) 4,6dimethyldibenzothiophene, (3) 4-ethyl, 6-methyldibenzothiophene, (4) 1,4,6-trimethyldibenzothiophene, (5) 4,6-diethyldibenzothiophene.
3.5. Adsorptive removal of nitrogen species in SRGO over ACF
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ACF which had been used to treat 80 ml of AD-HDS-SRGO prior to SRGO-treatment. Adsorptively treated SRGO (sulfur content: 11400 ppmS, nitrogen content: 40 ppmN) left only 40 ppmS while as supplied SRGO (sulfur content: 11780 ppmS, nitrogen content: 260 ppmN) did as much as 193 ppmS sulfur after HDS reaction over a commercially available CoMo catalyst even under the same reaction conditions. 3.6. Regeneration of activated carbon fiber After 80 ml AD-HDS-SRGO and 80 ml SRGO were treated, adsorbed sulfur and nitrogen species over used OG-20A (ACF) were extracted by various solvent. Sulfur and nitrogen content in the solvent was plotted in Fig. 10. 1-Methylnaphthalene extracted the largest amount of sulfur and nitrogen species from the ACF among the solvents. Toluene and tetralin could also extract sulfur and nitrogen species from OG-20A (ACF). Fig. 11 shows the removal amount of sulfur species in HDS-SRGO and nitrogen species in SRGO over the regenerated ACF. Regenerated
Fig. 11. Removal amount of sulfur species in HDS-SRGO and nitrogen in SRGO over virgin and regenerated ACF. Adsorbent: 0.3 g of OG-20A. Adsorption temperature: 30 8C. Feed: 30 ml of HDS-SRGO and SRGO.
ACF showed the same adsorption capacity for nitrogen in SRGO and sulfur species in HDS-SRGO.
4. Discussion The achievements of the present paper are summarized as followed. 1. Conventionally hydrodesulfurized gas oil carrying 50– 300 ppmS could be desulfurized to be less than 10 ppmS by adsorption over ACF at room temperature. 2. ACF bed could be used twice. ACF bed, which was used to adsorptively treat conventionally hydrotreated SRGO to be less than 10 ppmS, could further remove sulfur and nitrogen species of SRGO. Adsorptively treated SRGO showed very improved reactivity toward hydrodesulfurization. 3. Used ACF could be successfully regenerated by the aromatic solvent and extracted nitrogen and sulfur species from ACF with solvent can be recovered by distillation.
Fig. 10. Sulfur and nitrogen content of eluted solvents through exhausted ACF. Adsorbent: 0.3 g of OG-20A. Solvent-treating temperature: 70 8C. Solvent: toluene, 1-methyl-naphthalene and tetralin.
It is very critical task to assure the sulfur content of diesel oil to be less than regulated value, that is 10 ppmS in Euro IV. In general, refineries control the reaction temperature, space velocity and hydrogen/feed oil ratio to maintain the product specifications. However, the tolerance range in operating condition must be very narrow if the product specifications are ultimately limited. Hence, some kinds of back up facility should be installed to protect the run away of product quality, which is frequently encountered in the practical operation. Additional reactor, blending facility may be a solution for such emergency situation. The present authors suggest the adsorption bed as the back up facility to desulfurize the diesel having off-specification.
Y. Sano et al. / Fuel 84 (2005) 903–910
Adsorptive removal of sulfur species from the currently available hydrotreated gas oil (HDS-SRGO) of 300 ppmS to get the ultra low sulfur diesel described in the present paper may be impractical due to very short operation time to achieve 10 ppmS as shown in Fig. 4. However, deeply hydrotreated gas oil of 50 ppmS, which could be easily attained with the conventional HDS reaction when nitrogen species in SRGO are removed prior to HDS, showed much longer breakthrough time to reach 10 ppmS. Thus, the present process requires hydrodesulfurization of SRGO to be less than 50 ppmS in order to obtain the acceptable amount of adsorptively treated oil. Furthermore, early eluted gas oil can be used for fuel cell because it contained less than 1 ppmS. HDS of SRGO to be less than 50 ppmS could be achieved more easily by the pre-treatment to remove nitrogen species over ACF as described in our previous papers [19–22] and Fig. 7. Thus, ACF bed can be used consecutively for the adsorbent for the both of pre- and post-treatment steps. A particular temperature of 30 8C appears to absorb the largest amounts of sulfur species in HDS-SRGO over OG20A. Some of sulfur species, which are weakly adsorbed over ACF at 30 8C, appears to be desorbed at a higher temperature. On the other hand, it is not recommended to the present process to be operated at lower temperature than 30 8C due to the low fluidity of the gas oil at low temperature. It was found that addition of hydrocarbons having more than 10 carbons decreased the adsorption capacity of ACF. Especially, 1-methylnaphthalene of even 10% decreased the adsorption amount of sulfur to be a half. Such results indicate the limitation of the present process to treat the highly aromatic gas oil such as light cycle oil. Anti-solvent such as pressurized propane is worthwhile for trying to relieve the inhibition by aromatics, but application may be expensive due to high pressure for liquefying propane. ACF bed saturated with sulfur and nitrogen species could be successfully regenerated by conventional aromatic solvents as shown in Fig. 11. Naphthene can be also recommended as the solvent for regeneration of ACF. Hence, the life time of adsorbents, which is a very important factor in the practical operation, is believed to be long enough to minimize the operation cost. Repeated use of ACF does not lower the performance but reduce the cost of adsorbent. When the pre- and posttreatment steps have each adsorption bed, four adsorption beds must be installed (two for adsorption, two for regeneration) for continuous operation whereas the present integrated process needs three (two for two adsorption, one for regeneration) as shown in Fig. 12. This leads to the reduction of initial investment. The used ACF, which treated HDS-SRGO to keep the elutant less than 10 ppmS, showed almost the same breakthrough profiles of nitrogen species in SRGO as the virgin one. Repeated use of ACF reduces not only oil loss but also the frequency of
909
Fig. 12. Comparing the one bed system with two bed system.
regeneration, which indicates the good economy of present process. At the regeneration step, some amount of oil must be remained in the bed not to be lost. Loss of HDS-SRGO was reduced at its adsorptive removal to be included into SRGO, contributed the lower sulfur level of SRGO after HDS.
5. Conclusions A novel integrated process including adsorption system for pre- and post-treatment of gas oil was proved to achieve ultra deep desulfurization by using the OG-20A (ACF) as an adsorbent. Consecutive use of ACF for HDS-SRGO and SRGO reduced the number of adsorption bed, loss of feed oil and severity of HDS by removing the nitrogen and refractory sulfur species in SRGO. HDS-SRGO obtained by HDS is recommended to have less than 50 ppmS to be processed in the present adsorption system.
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