Ind. Eng. Chem. Res. 1994,33, 218-222
218
Hydrodesulfurization Reactivities of Various Sulfur Compounds in Diesel Fuel Xiaoliang Ma, Kinya Sakanishi, and Isao Mochida' Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816, Japan
The hydrodesulfurization (HDS) of a diesel oil was carried out in a batch autoclave reactor over the temperature range 280-420 "C for 0-90 min under a total pressure of 2.9 MPa, using CoMo and NiMo catalysts in both one and two stages. The HDS reactivities of benzothiophenes, dibenzothiophenes (DBTs), and their alkylated homologes existing in the diesel fuel were examined in detail by means of respective quantitative analyses. The sulfur compounds can be classified into four groups according to their HDS reactivities which were described by their pseudo-first-order rate constants. DBTs carrying two alkyl substituents a t the 4-and 6-positions, respectively, were the most resistant to desulfurization. H2S produced from reactive sulfur compounds in the early stage of the reaction is one of the main inhibitors for HDS of the unreactive species. A second stage using fresh hydrogen solved this inhibition problem, with NiMo achieving deeper desulfurization. 1. Introduction
Table 1. Properties and Composition of Diesel Fuel
Recently, closer attention has been paid to the deep desulfurization of diesel fuel since in the near future its sulfur content must be limited to 0.05 wt 5% to meet the regulation for environmental protection (Takatsuka et al., 1992). There are many kinds of sulfur compounds in diesel fuel, and their hydrodesulfurization (HDS) reactivities are very different (Sakanishi et al., 1992). In order to design the deep HDS process, it is essential to understand in detail their reactivities under practical desulfurization conditions. Some studies reported on the HDS of model sulfur compounds such as thiophene, benzothiophene (BT),and dibenzothiophene (DBT) (Singhal et al., 1981; Ho and Sobel, 1991;Miki et al., 1992)as well as some of their alkyl substituents (Houalla et al., 1980; Kilanowski et al., 1978; Miki et al., 1993). These HDSs of model sulfur compounds were always performed alone in the pure solvent. However, in practical hydrodesulfurization of diesel fuel, aromatic species as well as various types of sulfur compounds compete for active sites on the HDS catalyst surface. Moreover, H2S and hydrocarbons produced in the early stage of desulfurization, from some sulfur compounds with higher reactivity, probably influence HDS of less reactive sulfur compounds. Hence, the reactivities of various sulfur compounds in the diesel fuel need to be defined in the practical desulfurization process. In previous papers (Sakanishi et al., 1991, 1992, 1993; Maet al., 1994),the present authors proposed a multistage deep HDS of diesel fuel, which can reduce the total sulfur content of product oil to less than 300 ppm without fluorescent color development, and found that the NiMo catalyst appeared to have higher catalytic activity for HDS in the second stage than CoMo. In the present study, HDS of a diesel fuel was carried out in a autoclave reactor at temperatures of 280-420 "C for reaction times of 0-90 min under total pressures of 2.9 MPa, using CoMo and NiMo catalysts in one or two stages. The authors focused on the HDS reactivities of sulfur compounds existing in the diesel fuel which could suffer the influences of coexisting partners. Influences of H2S produced during HDS reaction on the reaction rate were also clarified. The catalytic activities of CoMo and NiMo in both the first and second stages were compared. 0888-5885/94/2633-0218$04.50/0
total sulfur (wt %) density (15 OC) (g/mL) pour point ("C) boiling range (ASTM) ("C) composition (vol % ) aromatics olefins saturates
0.706 0.84 -10 232-350 17 5 78
Table 2. Chemical Composition and Physical Properties of Catalysts CoMo
NiMo
chemical composition (%)" MOO3 coo NiO Si02
14.9 4.4
14.9
0.95 0.50
3.1 4.8 0.65
physical properties surface area (m2/g) pore volume (mL/g) shape average diameter average length
268 0.53 four leaves 1.4 X 1.2 2.8
273 0.52 four leaves 1.3 X 1.1 3.5
so4
a
Remainder A1203.
2. Experimental Section 2.1. Feed. The diesel fuel feed used in the present investigation was obtained from a Middle East crude with sulfur content of 0.706 wt % and boiling range from 232 to 365 "C. Its properties and composition are summarized in Table 1. 4,6-Dimethyldibenzothiophene(4,6-DMDBT) was synthesized according to Gerdil and Lucken (1965). 2.2. Catalysts. The catalysts were commercial CoMo/ A1203 and NiMo/AlzOs, supplied by Nippon Ketjen Co.. Their properties and compositions are listed in Table 2. The catalysts were presulfided with a 5% H2S/H2 flow under atmospheric pressure at 360 "C for 6 h at a heating rate of 120 "C/h before their use. 2.3. Reaction. HDS was performed in a 50-mL magnetically stirred (1000 rpm) batch autoclave in the temperature range of 280-420 "C at the catalyst-to-oil weight ratio of 0.1. The heating rate was ca. 25 "C/min, and the cooling rate was ca. 30 "C/min. The reaction time was counted from the moment when the temperature of the reactor reached the prescribed level. The total reaction pressure was controlled at 2.9 MPa throughout the reaction by adding gaseous hydrogen to compensate for its con-
0 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 219 sumption. The atomic ratio of hydrogen supplied to sulfur in the feed was more than 45, which was large enough to assure that the hydrogen partial pressure was approximately constant throughout the reaction. HDS of 4,6DMDBT as a model was run under prescribed reaction conditions using decalin as a solvent. 2.4. Analysis. The sulfur compounds in the feed and product oils were analyzed by a Yanaco gas chromatograph (G3800) equipped with a silicone capillary column (OV101: 0.25 mm X 50 m) and a flame photometric detector (FPD). The column temperature was programmed from 90 to 250 "C at a heating rate of 7.5 "Urnin, and then kept at 250 "C. Dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT), and 4,6-DMDBT in oil were identified with the standard samples, and other major peaks in GC were identified by comparing molecular weight (GCMS) or by referring to the relative retention times available in a report (Kagakuhin Kensa Kyokai, 1990). The sulfur content of sulfur compounds was quantified by GC-FPD according to the following equation:
where Ci is the sulfur content of sulfur compound i, Ai is the peak area of sulfur compound i, and f is a correction factor. The values of n and f were determined with standard samples. 3. Results 3.1. Composition and Distribution of Sulfur Compounds in the Diesel Fuel. The qualitative and quantitative analyses of the diesel fuel are summarized in Table 3, where sulfur contents (ppm) of the sulfur compounds are listed. All sulfur compounds were almost exclusively alkylbenzothiophenes and alkyldibenzothiophenes. The dominant sulfur-containing compounds found here were 4-MDBT, alkyldibenzothiophene-3 with two alkyl carbon atoms (C2-DBT-3), methyldibenzothiophene-1 (MDBTl),alkylbenzothiophene-4 with three alkyl carbon atoms (C3-BT-4),DBT, and 4,6-DMDBT, their sulfur contents being 209,194,173,171,169,and 146 ppm, respectively. 3.2. Reactivity of Sulfur Compounds under Different Conditions. Table 4 shows the changes in sulfur content of representative sulfur compounds by HDS at several temperatures for 20 min using the CoMo catalyst. Most alkylbenzothiophenes (e.g., C3-BT-5)exhibited high reactivity even at the low temperature of 280 "C, and were desulfurized completely at 360 "C. In contrast, alkyldibenzothiopheneswere more resistant to desulfurization. Most of them were still present in the desulfurized oil after reaction at 360 "C for 20 min. Among them, 4,6DMDBT was the most inactive, its sulfur content being 63 ppm even after reaction a t 420 "C. Nevertheless, the reaction temperature was noted to increase the HDS rates of all sulfur compounds as expected. In this temperature range the NiMo catalyst showed catalytic activity for desulfurization similar to that of the CoMo catalyst as shown in Table 4, although it appeared slightly less active at a higher temperature for the HDS of 4-MDBT. Figure l a shows the change in the sulfur content of representative sulfur compounds with time, using the CoMo catalyst at 360 "C. The dominant desulfurization took place within the first 20 min. All of the alkyl BTs were desulfurized completely by this time, except C3-BT4, C4-BT-7,C5-BT-3,C6-BT-3,and C7-BT-1,whose sulfur contents were 13,9,10,10, and 8 ppm, respectively. The alkyl DBTs were also desulfurized rapidly within the first 10 min; however, their HDS rates after 20 min decreased
Table 3. Sulfur Contents of Sulfur Compounds in Diesel Fuel and Their Pseudo-First-Order Reaction Constants. retention sulfur rate const (min-1) time sulfur contentin classifino. (rnin) compd feedb(ppm) CoMo NiMo cationc 1 15.73 MBT 11 1 >0.20 >0.20 2 17.36 C2-BT-1 1 47 >0.20 >0.20 1 3 17.67 C2-BT-2 35 >0.20 >0.20 1 4 17.82 C2-BT-3 74 >0.20 >0.20 1 28 5 17.97 C2-BTd >0.20 >0.20 122 0.26 0.39 1 6 18.13 C2-BT-4 1 46 >0.20 >0.20 7 19.27 C3-BT-1 1 75 >0.20 >0.20 8 19.41 C3-BT-2 1 106 >0.20 >0.20 9 19.68 C3-BT-3 0.091 0.093 2 171 10 19.97 C3-BT-4 11 20.03 C3-BT-5 1 0.25 >0.25 141 12 20.73 C4-BT-1 1 >0.20 >0.20 42 1 13 20.90 C4-BT-2 >0.20 >0.20 34 0.22 1 14 21.03 C4-BT-3 0.22 61 1 0.27 15 21.14 C4-BT-4 82 0.26 1 65 >0.20 >0.20 16 21.28 C4-BT-5 1 96 0.22 0.23 17 21.39 C4-BT-6 0.061 2 0.079 88 18 21.57 C4-BT-7 1 44 0.19 0.21 19 21.83 C4-BT-8 1 76 20 21.98 C4-BT-9 >0.20 >0.20 1 21 22.12 C4-BT-10 52 >0.20 >0.20 1 67 22 22.32 C4-BT-11 >0.20 >0.20 1 0.25 106 23 22.71 C5-BT-1 0.25 1 92 24 23.00 C5-BT-2 0.21 0.21 0.14 1 0.15 76 25 23.13 C5-BT-3 1 92 0.25 0.22 26 23.38 C5-BT-4 1 0.15 72 0.25 27 23.93 C6-BT-1 28 24.30 DBT 169 0.057 2 0.058 1 73 >0.20 >0.20 29 24.67 C6-BT-2 2 or 1 0.19 0.058 30 25.01 C6-BT-3 89 1 >0.20 >0.20 36 31 25.20 C6-BT-4 1 53 >0.20 >0.20 32 25.36 C6-BT-5 1 >0.20 >0.20 33 25.71 C6-BT-6 51 0.034 2 0.054 78 34 25.96 C7-BT-1 0.020 3 0.018 209 35 26.24 4-MDBT 0.065 2 0.063 173 36 26.60 MDBT- 1 57 37 26.74 MDBTd 0.071 0.058 2 38 27.05 MDBT-2 105 >0.20 >0.20 1 80 39 27.28 C7-BT-2 0.014 0.017 3 40 28.04 C2-DBT-1 89 0.006 0.008 4 41 28.27 4,6-DMDBT 146 0.030 0.031 3 42 28.71 C2-DBT-2 37 0.020 0.022 3 194 43 28.81 C2-DBT-3 0.064 0.062 2 44 29.08 C2-DBT-4 103 0.018 0.021 3 45 29.25 C2-DBT-5 133 0.034 0.032 3 46 29.54 C2-DBT-6 128 0.019 0.020 3 47 29.96 C3-DBT-1 77 0.007 0.010 4 48 30.18 C3-DBT-2 81 0.011 0.013 4 49 30.83 C3-DBT-3 120 0.021 0.020 3 65 50 31.09 C3-DBT-4 0.027 0.023 3 99 51 31.34 C3-DBT-5 0.011 0.012 4 96 52 31.69 C3-DBT-6 0.012 0.014 30r4 131 53 32.06 C3-DBT-7 0.022 0.022 3 54 32.54 C4-DBT-1 100 52 55 32.91 C4-DBT-2 0.024 0.024 3 87 56 33.11 C4-DBT-3 51 57 33.60 C4-DBT-4 57 58 33.86 C4-DBT-5 59 34.17 C4-DBTd 37 0.009 0.009 4 60 34.49 C5-DBT-1 61 66 61 35.21 C5-DBT-2 Reactionconditions: 360°C,2.9MPa. Sulfurcontentaofsulfurcontainingcompoundsin the feed. e Sulfur compoundswere claseified to four groups according to their HDS reactivities. d Not clear.
markedly. Specifically,the desulfurization of 4B-DMDBT became very slow, its sulfur content remaining at 60 ppm even after 90 min. 3.3. Reaction Rate Constants. In order to compare the reactivities of sulfur compounds, HDS behavior of each sulfur compound was analyzed according to the pseudo-first-order kinetic equation. Its integral rate equation is shown as follows:
220 Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 Table 4. Influence of Reaction Temperature on S u l f u r Contents of Representative S u l f u r Compounds in Diesel Fuel. reaction C3-BT-5 temr, (OC) CoMo NiMo feed 141 141 280 81 300 46 48 13 13 320 0 0 340 0 0 360 0 0 360 (H2S)b 380 0 0 0 0 400 0 420
DBT CoMo NiMo 169 169 156 118 118 93 89 55 54 30 24 55 45 a 10
4-MDBT CoMo NiMo 209 209 204 188 182 75 157 136 131 112 109 144 150 85 75 70 0 50 44
0 0
4,6-DMDBT CoMo NiMo 146 146 142 129 122 116 110 89 96 82 89 109 108 77 75 73 72 63
a Reaction conditions: 20 min, 2.9 MPa. Hydrogen sulfide partial pressure was 0.15 MPa at the beginning of the reaction.
200 y. n E
-5
P
160
I
4
C3-BT -5 DBT 4-MDBT 4.6DMDBT
c' v:
20
0
40 60 Reaction time (rnin)
-
0.06
k
M
0
40
80
60
loo
Reaction time (min)
Figure 1. Sulfur content and H2S partial pressure as function of time. Reaction conditions: 360 OC, 2.9 MPa, CoMo catalyst.
I
m C2-BT-4 . C3-BT - 5 C5-BT-I
A .
2
0
h
vu_'
DBT MDBT.1 C2-DBT-4
0 6MDBT
9
3 :
+
C2-DBT-3 C2-DBT-5 C3-DBT-5
0 4,6-DMDBT
4 .
C3.DBT-4
n C3-DBT-2
0
IO
20
30
40
50
60
70
Time (min)
Figure 2. Pseudo-first-order plots of some sulfur compounds. Reaction conditions: 360 OC, 2.9 MPa, NiMo catalyst.
Ln(Co/C,) = k t (2) where COand Ct are sulfur concentrations (ppm) of the sulfur compound at initial and reaction time t (rnin), respectively,and k is its rate constant (min-l). The pseudofirst-order plots of some major sulfur compounds on the NiMo catalyst are shown in Figure 2. Although some data for the first 10 min does not fit well, probably because of the influence of H2S produced during HDS, fairly good straight lines were obtained. Hence, the rate constants of major sulfur compounds at 360 "C over CoMo and NiMo catalysts, respectively, were calculated by the least-squares method and are listed in Table 3.
0
o
a
4
0
6
Time (min)
0
"U 0 2 0 4 0 6 0 Time (min)
Figure 3. Influence of different approaches on the second-stage HDS. The first stage conditions: 360 OC, 2.9 MPa, 30 min, CoMo catalyst. The second stage conditions: 360 "C, 2.9 MPa. (A) Neither hydrogen renewal nor catalyst renewal after the first stage (hydrogen sulfide partial pressure: 0.065 MPa). (B) Only hydrogen renewal. (C)Both hydrogen renewal and catalyst renewal using same catalyst (CoMo). (D) Both hydrogen renewalandcatalystrenewalusing NiMo catalyst.
The rate constants for most alkyl BTs on the CoMo catalyst were larger than 0.100 min-l (about 0.25 min-l), except C3-BT-4, C4-BT-7, C6-BT-3, and C7-BT-1, whose constants were 0.091, 0.079, 0.058, and 0.054 min-l, respectively. The rate constants of all alkyl DBTs were lower than 0.071 min-'. It is clear that the substituents attached at the 4- and/or 6-positions of DBT remarkably reduced their HDS reactivities, giving rate constants of 0.058,0.018, and 0.006 min-l for DBT, 4-MDBT, and 4,6DMDBT, respectively. Similar rate constants of 0.057, 0.020, and 0.008 min-', respectively, were observed with these species on the NiMo catalyst. 3.4. Influences of HzS on HDS. Figure l b shows the produced during HDS as a H2S partial pressure (PH~s) function of time. Within the first 5 min, a large amount of H2S was produced from the compounds with higher reactivities, such as alkyl BTs, rapidly increasing P Hto~ 0.043 MPa. After 10 min, P Happeared ~ almost constant. When the gas phase in the reaction system was replaced by fresh hydrogen after 30 min of the reaction, the HDS rate was remarkably enhanced by comparison with that > 0.055 MPa) as shown without hydrogen renewal (PH~s in Figure 3. The sulfur contents of 4-MDBT and 4,6DMDBT were reduced to 37 and 56 ppm, respectively, at 20-min reaction time, and their reductions were about 2.5 and 4.5 times larger than those in the case without hydrogen renewal. Catalytic activities of both CoMo and NiMo for HDS of alkyl DBTs were significantly retarded by H2S ( P H=~ 0.145 MPa, 5 % H2S at the beginning of the reaction) as shown in Table 4. The HDS conversions of DBT and alkyl DBTs decreased by 12-27% at 360 "C for 20 min. The conversions of 4,6-DMDBT in decalin were 64 and 4576, respectively, in the absence and presence of H2S ( P H ~=s 0.145 MPa) at the beginning of the reaction, exhibiting a significant inhibition by H2S. 3.5. Catalytic Activities of CoMo and NiMo. The NiMo catalyst was slightly inferior to the CoMo catalyst for HDS of sulfur compounds in the single stage, as shown in Table 3. In contrast, the NiMo in the successivesecond stage was significantlysuperior to CoMo as shown in Figure 3. HDS conversions of 4-MDBT and 4,6-DMDBT were 72 and 38%,respectively, over the NiMo, while they were only 48 and 25%, respectively, over the CoMo. The conversionsof 4,6-DMDBTin decalin were 51.6 and 38.6 %
Ind. Eng. Chem. Res., Vol. 33, No. 2, 1994 221 over the NiMo and CoMo, respectively, which coincided with those observed in the second stage. 4. Discussion HDS reactivities of heterocyclic sulfur compounds in a diesel fuel are governed basically by the types of C-S bonds and the position of alkyl substituents. The first factor is related to the strength of C-S bonds, and the second is related to the steric hindrance as well as the electron density on the sulfur atom (Nagai et al., 1986). According to the rate constants obtained, all of the MDBTs, except 4-MDBT, appeared to have reactivities similar to that of DBT, and all of the DMDBTs appeared to have reactivities similar to that of DBT or 4-MDBT, except 4,6-DMDBT, which shows the least reactivity of all of them. Only the substituents attached to 4- and 6-positions play an important role in dominating HDS reactivities of DBTs. Therefore, the sulfur compounds in diesel fuel are classified into four groups as follows. 1. The first group consists of most of the alkyl BTs, except C3-BT-4, C4-BT-7, and C7-BT-1 with the substituents probably at the 2- or/and 7-positions:
k > 0.10 min-' alkyl -:&!--alkyl 6
2
7
2. The second group consists of C3-BT-4, C4-BT-7, C7BT-1, DBT, and alkylated DBT homologes without substituents at the 4- and 6-positions:
k = 0.034-0.100 min-l
3. The third group consists of alkyl DBTs with one of the alkyl groups at either the 4- or 6-position:
k = 0.013-0.034 min-'
alkyl
4. The fourth group consists of alkyl DBTs with two of the alkyl groups at the 4- and 6-positions, respectively:
k = 0.005-0.013 min-' alkyl
--
GQ
0 0---alkyl alkyl
alkyl
The sulfur distribution in the four groups in the fuel is 39,20,26, and 15w t 5% ,respectively, and the average rate constants of the four groups at 360 OC and 2.9 MPa are about 0.25,0.058,0.020, and 0.007 min-1, respectively. The fourth group of sulfur compounds is the most difficult to desulfurize even at high temperatures or for a long reaction time. Therefore, it is necessary to pay closer attention to this group of sulfur compounds when the deep HDS of diesel fuel is considered.
Another point which the authors are concerned about is whether self-inhibition takes place in the HDS process. When the desulfurization rate process is carefully examined, the reaction is very fast up to 10 min and then becomes gradually slower, and the DBTs of lower reactivities have more severe retardation in the latter stage. There are two causes for such a retardation: (1)inhibition of HzS produced by HDS of reactive species and (2) inhibition of the olefinic or aromatic products with strong adsorption. The HzS produced by HDS was found to be one of the main inhibition factors, especially in deep HDS as exhibited by comparing the HDS reactivities of 4,6DMDBT in both oil and decalin in the presence and absence of H2S. This also explains why some data did not fit the pseudo-first-order reaction well within the first 10 min of the reaction as shown in Figure 2. It is also interesting that the NiMo catalyst exhibited higher catalytic activity than CoMo for HDS of alkyl DBTs in both the second stage and the model experiment. Generally speaking, NiMo has a higher hydrogenation activity than CoMo, which is probably favorable for HDS of alkyl DBTs by means of hydrogenating a benzene ring first. However, the same result was not observed in the first-stage HDS. Since PHB produced by HDS in the second stage and in the model experiment (0.006and 0.004 MPa, respectively) was much less than that in the first stage (0.06 MPa), it is considered that H2S produced in initial HDS is responsible for reducing the activities of the catalysts for HDS, especially the NiMo catalyst. Such results explain why the two-stage HDS using CoMo and NiMo successively with fresh hydrogen is superior to the one-stage HDS as reported in our earlier papers (Sakanishi et al., 1993; Ma et al., 1994).
5. Conclusions The sulfur compounds in the diesel fuel can be classified into four groups according to their HDS reactivities: (1) most of the alkyl BTs, (2) DBT and alkyl DBTs without substituentsat the 4- and 6-positions, (3) alkyl DBTs with only one of substituents at either the 4- or 6-position, and (4) alkyl DBTs with two of the alkyl substituents at the 4- and 6-positions, respectively. The pseudo-first-order rate constants of four groups are about 0.25,0.058,0.020, and 0.007 min-l, respectively. The fourth group is the most difficult to desulfurize. The H2S produced from alkyl BTs in the initial reaction is one of the main inhibitors for deep HDS of unreactive species. The NiMo catalyst exhibits higher catalytic activity than the CoMo for HDS of alkyl DBTs under low H2S partial pressure (
