Ind. Eng. Chem. Res. 1997, 36, 1519-1523
1519
Hydrodesulfurization of Catalytic Cracked Gasoline. 1. Inhibiting Effects of Olefins on HDS of Alkyl(benzo)thiophenes Contained in Catalytic Cracked Gasoline S. Hatanaka*,† and M. Yamada Department of Applied Chemistry, Faculty of Engineering, Tohoku University, Aoba, Aramaki, Aoba-ku, Sendai 980-77, Japan
O. Sadakane Petroleum Research Laboratory, Mitsubishi Oil Company Ltd., 4-1 Ohgimachi, Kawasaki-ku, Kawasaki, 210 Japan
The HDS reaction feature of CCG on Co-Mo/γ-Al2O3 was investigated in detail to make clear the important factors for the deep HDS of CCG. Two kinds of CCG, which are produced from different origins and contain 229 and 61 ppm sulfur, respectively, were examined. GC-AED analyses showed that both CCG contain 11 alkylthiophenes, 2 alkylbenzothiophenes, 3 alkylthiacyclopentanes, and 2 disulfides. In the reaction at 210 °C, 1.6 MPa using a conventional flow reactor of pilot scale, the total sulfur content decreased from 229 to 127 ppm, whereas 22 ppm of thiols was produced. The HDS reactivities of alkylbenzothiophenes were higher than those of alkylthiophenes, and HDS reactivity decreased with increasing the number of alkyl groups. The reactivities of these sulfur compounds in CCG were much lower than those of the corresponding pure compounds obtained by the individual reaction of pure sulfur compounds. The low reactivity of sulfur compounds in CCG was found to be ascribed to the inhibiting effect of olefins in CCG by comparing the HDS reactivity of thiophene in different solvents including olefins. The inhibiting effect of olefin seems to be caused by two types of adsorption of olefin, that is, weak adsorption and strong adsorption. Introduction Recently, the process development to meet the severe limitation of sulfur content in fuel oils is a very important theme. Every effort has been extensively made for understanding the reactivities of individual sulfur components in feedstocks to be treated and the HDS catalysis. For example, with respect to the HDS mechanism, various mechanisms have been proposed using pure sulfur compounds (e.g., thiophene, benzothiophene, and dibenzothiophene) (Kwart et al., 1980; Prins et al., 1989), since a one-point mechanism was proposed (Lipsch and Schuit, 1969). With respect to the fine structure of the catalyst active sites, a Co-Mo-S model has been proposed (Topsøe and Clausen, 1984) using some sophisticated techniques (Mossbauer and EXAFS) and widely accepted. However, in contrast to the detailed discussions about the HDS mechanisms of pure sulfur compounds, the details of a hydrotreating mechanism of petroleum feedstocks remain unknown. For example, the studies about HDS of catalytic cracked gasoline (CCG) (Desai et al., 1994), hydrotreating of olefin-containing naphtha (Satchell and Crynes, 1975; Ali and Anabtawi, 1995) and LCO (Baron et al., 1992), and HDS of pyrolytic gasoline (Casagrande et al., 1955; Meerbott and Hinds, 1955) suggest that the HDS features of sulfur compounds in the hydrotreatment of a petroleum fraction are different from those of individual sulfur compounds. One of the reasons for these differences seems to be caused by the effects of various compounds contained * Corresponding author. Telephone: +81-44-344-3128. Fax: +81-44-344-3645. † Present address: Petroleum Research Laboratory, Mitsubishi Oil Co. Ltd., 4-1 Ohgimachi, Kawasaki-ku, Kawasaki, 210 Japan. S0888-5885(96)00377-6 CCC: $14.00
in the petroleum fraction. While inhibiting effects of hydrogen sulfide have been extensively studied (Satterfield and Roberts, 1968; Metcalfe, 1969; Kasahara et al., 1995), a few studies have been reported on the effects of hydrocarbons on the HDS reaction. An inhibiting effect of heptane on HDS of five kinds of sulfur compound mixtures (dimethyl disulfide, diethyl disulfide, benzenethiol, thiacyclopentane, and thiophene) at 370 °C and 20-30 MPa has been reported (Phillipson, 1971). It has been reported that the presence of cis-2butene does not have any particular influence on thiophene HDS on a single crystal of Mo at 340 °C and atmospheric pressure (Bussell and Somorjai, 1987). An inhibiting effect of naphthalene or tetralin on HDS of 4,6-dimethyldibenzothiophene at 2.5 MPa has been observed (Isoda et al., 1994). The present work tries to make clear the important factors which affect the hydrotreatment of CCG and also to make clear the difference between the HDS feature of CCG and that of an individual sulfur compound. For this, we have characterized the sulfur compounds contained in CCG and compared the HDS reactivities of the sulfur compounds in CCG with those of individual sulfur compounds dissolved in some model compounds. Experimental Section Feedstock. (a) CCG. CCG-A produced from low sulfur atmospheric residue and CCG-B produced from desulfurized vacuum gas oil were used here. (b) Pure Sulfur Compounds. Commercial-grade thiophene, 2-methyl- or 3-methylthiophene, 2,5-dimethylthiophene, 2-ethylthiophene, and benzothiophene without further purification were dissolved in toluene at 2.83 × 10-4 mol/mol of toluene and supplied for activity tests. This concentration of sulfur compounds corresponds to © 1997 American Chemical Society
1520 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 Table 1. Physical and Chemical Properties of the HDS Catalyst shape support metal content, wt % MoO3 CoO a
1/ 22
in. trilobe r-Al2O3
15.0 4.6
surface area, m2 g-1 total pore volume, cm3 g-1 bulk density, g cm-3
200 0.61 0.63 (0.68)a
0.6-1.0 mm crushed catalyst for microreactor.
100 wt ppm of sulfur content. Thiophene dissolved in n-heptane, n-octane, or a mixture of toluene (80 wt %) and olefins (20 wt %) was also supplied for activity tests. Olefins used here were commercial-grade 1-octene and diisobutylene without further purification. Analyses. (a) CCG and CCG-HDS Products. Feedstock and HDS products were analyzed by the following methods. The contents of total sulfur and thiol-type sulfur were measured by oxidative microcoulometry (ASTM-D3120) and a potentiometric method (ASTM-D3227), respectively. Sulfur compounds were quantitatively analyzed by a GC-AED (Hewlett Packard 5921A) equipped with a 50 m PONA column. Identification of sulfur compounds was performed by GC-MS. Olefin types were measured by 1H-NMR. (b) Pure Sulfur Compounds and Their HDS Products. In addition to the methods used in a, GC and PIONA-GC (Analytical Controls Inc.) were used for the measurement of hydrogenation reactivity of the olefins in the reaction of thiophene/toluene/olefin systems. Catalyst. CoO-MoO3/γ-Al2O3 catalyst was prepared by a conventional pore-filling method using a prescribed amount of Mo and Co aqueous solutions. CoO and MoO3 contents are 4.6 and 15.0 wt %, respectively. After drying at 110 °C for 12 h, the catalyst was calcined at 550 °C for 1 h. A total of 60 mL of the catalyst was packed in a bench pilot reactor, and 4 mL of the catalyst crushed to 0.6-1.0 mm particles was packed in a
Figure 1. Sulfur compounds contained in CCG-A by GC-AED.
Table 2. Properties of CCG composition,a vol % saturates aromatics olefins (olefin structure breakdownb) 1-olefins H2CdCR2 H2CdCHR internal olefins HRCdCR2 HRCdCHR total sulfur,c ppm density, g/cm3 @ 15 °C distillation temperature,d °C IBP 10% 30% 50% 70% 90% EP research octane number
CCG-A
CCG-B
41.9 27.7 30.4
44.6 33.6 21.8
2.3 5.0
2.2 3.0
9.8 13.3 229 0.778
8.6 8.0 61 0.780
48 88 110 136 165 201 231 87.0
68 90 111 134 146 174 205 86.7
a By ASTM D-1319. b Type ratio was calculated by 1H-NMR data on the assumption that R2CdCR2 type olefin was not included. NMR peak assignment for the olefin types, ppm by TMS standard: H2CdCR2, 4.50-4.80; H2CdCHR, 4.80-5.10 and 5.606.00; HRCdCR2, 5.10-5.25; HRCdCHR, 5.25-5.60. c By ASTM D-3120. d By ASTM D-2887.
microreactor. Metal composition and physical properties of the catalyst are shown in Table 1. HDS Reaction Procedure. Two conventional fixedbed flow reactors (a bench pilot reactor of 1 in. i.d. and a microreactor of 1/2 in. i.d.) were used for the catalyst presulfiding and HDS activity tests. CoO-MoO3/γAl2O3 catalyst was presulfided at 250 °C for 2 h and then 300 °C for 2 h in the stream of a dibutyl disulfide/ straight run naphtha (bp 80-160 °C, total sulfur 250 wt ppm)/hydrogen mixture. Following the presulfiding, in order to stabilize the activity, the catalyst was aged at 300 °C for 48 h in the stream of a straight run
Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1521 Table 3. Sulfur Compounds in CCG-A before and after HDSa reaction temperature, °C feedstock
210
sulfur compound
sulfur, ppm
sulfur, ppm
thiophene thiacyclopentane 2-methylthiophene 3-methylthiophene 2-methylthiacyclopentane 3-methylthiacyclopentane + 2-ethylthiophene 2,3-dimethylthiophene 2,4-dimethylthiophene 2,5-dimethylthiophene + 3-ethylthiophene 3,4-dimethylthiophene C3-thiophenes C4-thiophenes benzothiophene methylbenzothiophenes dimethyl disulfide diethyl disulfide unknown thiols total sulfur
7 5 11 10 3 5 5 7 4 3 15 7 63 63 4 1 16 0 229
4 2 7 5 3 5 3 5 2 1 15 6 2 28 1 1 15 22 127
a
220
conversion, % 43 60 36 50 0 0 40 29 50 67 0 14 97 56 75 0 6 45b (54)c
sulfur, ppm 3 1 6 3 2 4 2 4 2 1 12 5 0 12 0 0 7 17 81
conversion, % 57 80 46 70 33 20 60 43 50 67 20 29 100 81 100 100 56 65b (72)c
Reaction conditions: pressure, 1.6 MPa, LHSV, 3.5 h-1, H2/feed ratio, 338 N L/L. b HDS %. c Conversion of sulfur compounds.
naphtha/hydrogen mixture. After these catalyst pretreatments, the feedstocks were flown into the reactors with hydrogen. The reaction conditions were as follows: 150-225 °C, 1.3-1.6 MPa, LHSV 3.5 h-1, H2/feed ratio 338 N L/L. Result and Discussion Characterization of Sulfur Compounds in Feedstocks. The properties of two kinds of CCG examined here are analyzed and are shown in Table 2. It is important to acquire the detailed information about the sulfur compounds contained in the feedstocks. As shown in Table 2, CCG-A contains 229 ppm sulfur and CCG-B contains 61 ppm sulfur. The GC-AED analysis of CCG-A is shown in Figure 1. Eleven alkylthiophenes, 2 alkylbenzothiophenes, 3 alkylthiacyclopentanes, and 2 disulfides are observed. These identification results basically agree with a report analyzing CCG containing 1000 wt ppm sulfur (Albro et al., 1994). The compositions of these sulfur compounds in CCG-A are shown in Table 3. The total amount of alkylthiophenes is about half that of alkylbenzothiophenes. CCG-B also contains the same sulfur compounds. As shown in Table 3, CCG contains olefins (20-30 vol %), saturates (paraffins, naphthenes) (40-45 vol %), and aromatics (about 30 vol %). 1H-NMR analyses show that olefin types in CCG-A and CCG-B are similar to each other. That is, about 80% of the olefins contained in both CCG are internal type. Hydrodesulfurization of CCG. The compositions of sulfur compounds in the HDS products of CCG-A reacted in a bench pilot plant are also shown in Table 3. As shown in this table, after HDS the total sulfur content clearly decreases, whereas thiols are produced. In order to compare the overall HDS reactivity of CCG-A with that of CCG-B, HDS (%) and conversion (%) are defined in the following equations respectively considering and removing the formation of thiols.
HDS (%): (1 - product sulfur/feed sulfur) × 100 conversion (%): {1 - (product sulfur thiol sulfur)/feed sulfur} × 100
Figure 2. HDS conversion of individual sulfur compounds and CCG-A. Reaction conditions: pressure, 1.3 MPa (CCG-A, 1.6 MPa); LHSV, 3.5 h-1; H2/feed ratio, 338 N L/L. (a) Alkylthiophene concentration 2.83 × 10-4 mol/mol of toluene (100 ppm sulfur in toluene). (b) Thiol yield was calculated into conversion. (c) Bench pilot plant data.
The calculated results are plotted in Figure 2. It is noted that the difference of the reactor scale (bench pilot reactor or microreactor) does not affect the results, suggesting the high reliability of the present results. As shown in this figure, removing the effect of thiol formation, the HDS reactivity of CCG-A is not so different from that of CCG-B. The following points are suggested from Table 3. Alkylbenzothiophenes are more reactive than alkylthiophenes. It is noted that the reaction rate constant of benzothiophene is 6 times larger than that of
1522 Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 Table 4. Effects of the Hydrocarbons on Thiophene HDSa hydrocarbon
reaction condition
HDS, %
thiophene conversion, %
sulfur conversion to thiol + sulfide, %
hydrogenation of olefin, %
toluene n-heptane toluene n-octane toluene (80 wt %) + 1-octene (20 wt %) toluene (80 wt %) + diisobutyleneb (20 wt %)
A A B B B B
68.8 68.9 52.8 54.8 0.5 9.4
68.8 68.9 52.8 54.8 20.0 19.2
0 0 0 0 19.5 9.8
18.8 3.5
a Reaction conditions: (A) thiophene concentration, 1.56 × 10-3 mol/mol; temperature, 180 °C; pressure, 1.3 MPa; H /feed ratio, 2.33 2 mol/mol; catalyst/feed, 0.48 g of cat‚min/mol. (B) thiophene concentration, 2.83 × 10-4 mol/mol; temperature, 150 °C; pressure, 1.3 MPa; H2/feed ratio, 1.60 mol/mol; catalyst/feed, 0.34 g of cat‚min/mol. b Composition: 2,4,4-trimethyl-1-pentene, 74.9 mol %; 2,4,4-trimethyl2-pentene, 20.6 mol %; others, 4.5 mol %.
thiophene. With increasing the number of alkyl substitution groups, the HDS reactivity decreases. It is also noted that 2-methylthiophene is less reactive than 3-methylthiophene, being due to the steric hindrance of a methyl group substituted at the 2-position of thiophene. Thiacyclopentane and disulfides are extremely reactive. These reactivities observed here may be different from their intrinsic HDS reactivity. In order to understand the important factors which affect the HDS reactivity of CCG, the intrinsic HDS reactivity of pure sulfur compounds was investigated and compared with the above feature in the following section. Hydrodesulfurization of Pure Sulfur Compounds. In order to observe the intrinsic reactivity of sulfur compounds contained in CCG, HDS reaction of individual sulfur compounds dissolved in toluene was carried out at several temperatures. The obtained conversions are shown in Figure 2. From this figure, it is clear that the reactivity decreases as benzothiophene > thiophene > 3-methylthiophene > 2-methylthiophene > 2-ethylthiophene > 2,5-dimethylthiophene within the temperature range examined. It is noted that this order of reactivity is the same as that of sulfur compounds in CCG HDS. The reaction rate constant of benzothiophene is 2 times larger than that of thiophene, and this result is different from the results of CCG HDS, in which the reaction rate constant of benzothiophene is 6 times larger than that of thiophene. This difference may be caused by the effects of competitive adsorption of various sulfur compounds and hydrocarbons contained in CCG. It is also noted that no formation of thiol was observed. Therefore, thiol produced in CCG HDS is not the intermediate of the HDS of sulfur compounds. Comparing the HDS conversion shown in Figure 2 with that of the corresponding sulfur compounds in CCG HDS, it is noticeable that the conversion in the individual reaction is much higher than that in the CCG HDS. In other words, the intrinsic reactivity of individual sulfur compounds is depressed in CCG HDS. In the following section, the reason for the low reactivity of sulfur compound in CCG HDS was studied in more detail. Effects of CCG Hydrocarbons on HDS. In order to investigate the factors which depress the intrinsic reactivity of sulfur compounds contained in CCG as mentioned above, the effects of the CCG hydrocarbons on HDS were studied. As is already observed in the first section, the major components of CCG are paraffins, olefins, and aromatics. The effects of these compounds on HDS reactivity of thiophene were examined by using thiophene dissolved in toluene, n-heptane, n-octane, 1-octene/toluene, or diisobutylene/toluene as reaction feeds. The obtained results are shown in Table 4. As shown in Table 4, it is noted that the total sulfur
Figure 3. Response of HDS % in the presence of olefin. Reaction conditions: temperature, 150 °C; pressure, 1.3 MPa; H2/feed ratio, 1.60 mol/mol; catalyst/feed, 0.34 g of cat‚min/mol. Feed: thiophene concentration, 2.83 × 10-4 mol/mol in toluene or in 80 wt % toluene + 20 wt % diisobutylene.
content does not decrease only when olefins are present. In other words, HDS of thiophene does not proceed in the presence of olefin. It is also noted that the decreases of thiophene conversion are accompanied by the formation of thiols and sulfides. The presence of each olefin clearly depresses the HDS reactivity of thiophene in two ways, that is, thiol production and inhibiting effects of olefins on HDS active sites. Inhibiting Effect of Olefin on HDS. In order to study the reason for the inhibiting effect of olefins in more detail, HDS activity tests of thiophene in the presence of olefins were studied further. After the HDS reaction of thiophene reached to its steady state, the feed was switched from thiophene/toluene to thiophene/ diisobutylene/toluene. The change of thiophene HDS accompanied by the change of the feed is shown in Figure 3. As shown in this figure, olefin strongly inhibits the thiophene conversion only when the feed contains olefin. The depressed conversion is almost recovered when the feed is switched back to the feed without olefin. As shown in Figure 3, when the olefincontaining feed is flown into the reactor again, the thiophene conversion responded in the same way as the first time. That is, the inhibiting effect of olefins is almost reversible. However, a small portion of conversion loss remained after the feed was switched back to the feed without olefin. Then, the reaction temperature was swung to 200 °C for a moment. As is also shown in Figure 3, by the temperature swing, the small conversion loss remained after the change of feed was recovered. The above results suggest that olefin has two kinds of inhibiting effects. One is reversible and may be due to equilibrium adsorption. The other is irreversible and may be due to oligomeric compounds which are easily desorbed at higher temperature.
Ind. Eng. Chem. Res., Vol. 36, No. 5, 1997 1523
In HDS of CCG, lower conversion of olefin hydrogenation as well as higher conversion of HDS is very important to keep the octane value high. Concerning this problem, it has been reported that olefin is easily hydrogenated and the octane value fatally drops at higher pressure and temperature reaction conditions (Desai et al., 1994). Their report suggests that it is not easy to keep the octane value high in HDS of CCG at higher severity conditions. Therefore, the present experiment was undertaken to investigate the possibility of deep HDS under lower severity conditions. As a result, it was first found that olefin strongly inhibits HDS of CCG. The present results suggests that new catalyst with high HDS selectivity is necessary for deep HDS of CCG under lower severity conditions, keeping the octane value high. Literature Cited Albro, T. G.; Dreifuss, P. A.; Wormsbecher, R. F. Quantitative Determination of Sulfur Compounds in FCC Gasolines by AED. J. High Resolut. Chromatogr. 1993, 16, Jan, 13. Ali, S. A.; Anabtawi, J. A. Olefins can limit desulfurization of reformer feedstock. Oil Gas J. 1995, July 3, 48. Baron, K.; Miller, R. E.; Tang, A.; Palmer, L. Hydrotreating of Light Cycle Oil. NPRA Annual Meeting, New Orleans, LA, 1992; Paper AM-92-20. Bussell, M. E.; Somorjai, G. A. A Radiotracer (14C) and Catalytic Study of Thiophene Hydrodesulfurization on the Clean and Carbided Mo(100) Single-Crystal Surface. J. Catal. 1987, 106, 93. Casagrande, R. M.; Meerbott, W. K.; Sartor, A. F.; Trainer, R. P. Selective Hydrotreating over Tungsten Nickel Sulfide Catalyst. Treatment of Cracked Gasolines. Treatment of Cracked Gasolines. Ind. Eng. Chem. 1955, 47, April, 744. Desai, P. H.; Lee, S. I.; Jonker, R. J.; De Boer, M.; Vrieling, J.; Sarli, M. S. Reduce Sulfur in FCC Gasoline. Fuel Reformulation 1994, Nov/Dec, 43. Isoda, T.; Ma, X.; Mochida, I. Reactivity of Refractory Sulfur Compounds in Diesel Fuel (part 2). Inhibition of Hydrodesulfu-
rization Reaction of 4,6-Dimethyldibenzothiophene by Aromatic Compound. Sekiyu Gakkaishi 1994, 37, 506. Kasahara, S.; Koizumi, N.; Iwahashi, J. K.; Yamada, M. Effect of Fe, Co, Ni on Hydrodesulfurization Activity of Sulfided Mo/Al2O3 (Part 1). Sekiyu Gakkaishi 1995, 38, 345. Kwart, H.; Schuit, G. C. A.; Gates, B. C. Hydrodesulfurization of Thiophenic Compounds: The Reaction Mechanism. J. Catal. 1980, 61, 128. Lipsch, J. M. J. G.; Schuit, G. C. A. The CoO-MoO3-Al2O3 Catalyst. Part 3. Catalytic Properties. J. Catal. 1968, 15, 179. Meerbott, W. K.; Hinds, G. P., Jr. Reaction studies with Mixtures of Pure Compounds. Selective Hydrotreating over Tungsten Nickel Sulfide Catalyst. Ind. Eng. Chem. 1955, 47, April, 749. Metcalfe, T. B. Inhibition par le sulfure d’hydroge`ne de la re´action d’hydrode´sulfuration des produits pe´troliers. Chim. Ind. Gen. Chim. 1969, 102, Nov, 1300. Phillipson, J. J. Kinetics of Hydrodesulfurization of Light and Middle Distillates. AIChE National Meeting, Houston, Feb 1971; Paper 31b. Prins, R.; De Beer, H. J.; Somorjai, G. A. Structure and Function of the Catalysis and the Promoter in Co-Mo Hydrodesulfurization Catalysts. Catal. Rev.-Sci. Eng. 1989, 31 (1 & 2), 1. Satchell, D. P.; Crynes, B. L. High olefins content may limit cracked naphtha desulfurization. Oil Gas J. 1975, Dec 1, 123. Satterfield, C. H.; Roberts, G. W. Kinetics of Thiophene Hydrogenolysis on a Cobalt Molybdate Catalyst. AIChE J. 1968, 14, Jan, 159. Topsøe, N. Y.; Topsøe, H. Characterization of the Structures and Active Sites in Sulfided Co-Mo/Al2O3 and Ni-Mo/Al2O3 Catalysts by NO Chemisorption. J. Catal. 1983, 84, 386. Topsøe, H.; Clausen, B. S. Importance of Co-Mo-S Type Structures in Hydrodesulfurization. Catal. Rev.-Sci. Eng. 1984, 26 (3 & 4), 395.
Received for review July 2, 1996 Revised manuscript received January 7, 1997 Accepted January 13, 1997X IE9603777 X Abstract published in Advance ACS Abstracts, February 15, 1997.
