Ind. Eng. Chem. Res. 1990,29, 1793-1801
1793
Properties of Fluid Catalytic Cracking Decant Oils of Different Origins in Their Single Carbonization and Cocarbonization with a Petroleum Vacuum Residue Yasuhiro Nesumi, Takashi Oyama, Yoshio Todo, and Akemi Azuma Marifu Refinery, Koa Oil Co. Ltd., Waki-Cho, Yamaguchi 740, Japan
Isao Mochida* and Yozo Korai Institute of Advanced Material Study, Kyushu University 86, Kasuga, Fukuoka 816, Japan
Eight decant oils of fluid catalytic cracking (FCC-DO) were compared in terms of their structure and carbonization properties in their single carbonization and cocarbonization with a low sulfur vacuum residue (LSVR) to reveal structure-reactivity correlation in the needle coke production. Although eight oils all provided (at 500 "C under 16 kg/cm2 of pressure) lumps of needle cokes in a tube bomb, their coefficients of thermal expansion (CTE) ranged from -0.15 X lo4 (shrink, oil E) to 0.6 X lo* OC-l (oil A). Their cocarbonization with LSVR at 480 "C under 8 kg/cm2 provided lump cokes of variable quality in terms of CTE and amount of bottom mosaic cokes. It should be noted that the best FCC-DO in its single carbonization was not always a better partner in the cocarbonization. In the cocarbonization, the best FCC-DO (G) gave a needle coke with the low CTE of 0.6 X 10* OC-l and no bottom mosaic cokes while the worst oil (A2) gave a CTE of 1.2 X lo4 OC-l and bottom mosaic coke 1.4 mm thick. The oils consisted principally of saturate and light and heavy aromatic fractions, the contents of which varied from one oil to another. Among the fractions, the aromatic fractions were the major source of the coke, as indicated by a fair correlation of their content and aromaticity with their coke yield in the single carbonization. The saturate fraction appears to participate in the carbonization reaction as a poor solvent for the mesophase formation. The oil of the better partner in the cocarbonization was less paraffinic, and its aromatic fraction carried some alkyl groups of rather short chains. T h e roles of FCC-DO in the cocarbonization, particularly in the formation of bottom mosaic texture, are discussed, emphasizing the importance of the dissolution of the mesophase derived from the most reactive portion of asphaltene in the residue.
Introduction Needle cokes of better quality have been continuously requested for electrodes of better performance. Fluid catalytic cracking decant oil (FCC-DO)has been recognized as the best petroleum feedstock or partner with vacuum residue (VR) for the needle coke produced in the commercial delayed coker (Stocks and Guercio, 1985). FCC-DO is fairly aromatic, and its moderate molecular size with a considerable content of alkyl groups and naphthenic hydrogens allows the well-oriented flow texture in the coke at a reasonable yield (Mochida et al., 1988a), although it carries also a significant amount of long paraffins and sometimes causes the formation of mosaic coke of lower grade at the bottom of the coker (Marsh and Walker, 1979; Mochida et al., 1989; Nesumi et al., 1989). Recently, increasing numbers of crudes and demand for gasoline have produced a variety of FCC-DOs of variable natures. Hence, their exact evaluation and the relation between their structural characteristicsand carbonization properties are most valuable for the cokers of the present petroleum refinery. In the present study, a series of FCC-DOs supplied from refineries in Japan were studied in terms of their structural characteristics and properties in the single carbonization and cocarbonization with VR of low sulfur crude (LSVR). The carbonization was carried out in a tube bomb which has been reported to prepare a needle coke lump comparable to the commercial one from the same feedstock in terms of CTE (coefficientof thermal expansion), optical anisotropy, and density (Mochida et al., 1986, 1987a, 1988a,b; Eser and Jenkins, 1989). Their structure depends strongly on the kind of crude oils and severity of the cracking conditions, and they exhibit a variety of carbonization properties. Hence, the OS8S-5S85/90/2629-1~93$02.50/0
correlation between their structure and carbonization properties may suggest a requirement for feed in the single carbonization and a partner in the cocarbonization to produce better coke of uniform quality in the delayed coker.
Experimental Section Eight FCC-DOs of different origins supplied from refineries in Japan were used in the present study. The general properties of DOs and a low sulfur petroleum vacuum residue (LSVR) are summarized in Table 1. Residual catalysts seem to be removed in refineries. A sample of ca. 40 g was carbonized in a stainless steel tube bomb that was heated in a sand bath. The heating rate was about 250 OC/min. The carbonization temperature was fixed at 500 "C, and the carbonization pressure was adjusted to be atmospheric or 16 kg/cm2 by the initial nitrogen pressure and adequate purging through a control valve during the carbonization. The lump coke recovered from the bomb was examined microscopically, and the CTE of the lump was measured after the calcination at 1000 "C. Details are described in previous papers (Mochida et al., 1976, 1986, 1987a). The cocarbonizationwas performed at 460-500 "C under a fixed pressure of 8 kg/cm2. The resultant lump cokes were examined microscopically and by CTE measurement. Some lumps carrying bottom mosaic texture were measured after cutting off the bottom portion. Experimental details of cocarbonization are described in previous papers (Mochida et al., 1989). Structural analyses of whole DOs were done by 'H NMR measurement and elemental analysis, carbon aromaticity being calculated according to the modified Brown-Ladner method (Iwata et al., 1980). DOs were also divided into 0 1990 American Chemical Society
1794 Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 Table I. Properties of Feedstocks A1 1.038 SP 102.8 viscosity, cSt elem. anal., YO C
H N S 0 Conradson C, YO fab UC CsideC
composition, % saturate aromatic light heavy resin asphaltene a
A2
B
C
D
E
F
G
LSVR
1.005 45.35
1.065 12.98
1.0372 25.4
0.9881 25.1
0.9621 24.6
0.9954 16.1
0.9930 13
0.928 74.8
89.60 9.05 0.06 0.87 0.42 5.54 0.62 0.36 14.80
89.52 9.81 0.08 0.34 0.25 4.45 0.58 0.34 37.25
90.63 7.80 0.10 1.08 0.39 0.69 0.36 9.39
89.76 8.37 0.22 1.38 0.27 4.12 0.67 0.35 6.77
88.90 9.93 0.19 0.43 0.55 4.03 0.57 0.36 24.20
89.19 10.43 0.07 0.12 0.19 2.50 0.55 0.33 78.77
89.86 9.38 0.14 0.35 0.27 1.66 0.61 0.35 13.78
89.64 9.50 0.04 0.35 0.47 2.83 0.59 0.36 33.06
86.2 12.5 0.4 0.2 0.7 10.2 0.18 0.52
23
30
4
8
27
39
31
36
52
52 24 1 0
55 14 1 0
44 51 1 0
75 16 1 0
50 22 1 0
27 33 0 0
43 25 1 0
10 52 2 0
27 11 10
Specific gravity. *Aromaticity. See Table 111.
Table 11. Coke Yield and CTE of Lump Cokes
coke yield, YO 1 kg/cm2 a
16 kg/cm* a
CTE," 104/'C coke gradingd
D
E
F
G
9 12 51 61 0.21
6 16 51 48 0.30
3 23 43 38 -0.15c
4 16 32 38 0.13
5 34 42 39 0.52
E
G
E
E
R
24 1.2 0.3
23 0.9 1.0'
22 0.9 1.2
0.2 1.1
A1
A2
B C Single Carbonization
13 17 50 50 0.61
22 10 47 46 0.47
31 37 76 65 0.42
R
R
G
29 0.9 0.6h
20 1.2 1.4
Cocarbonization' yield, 70 C T E j 104/'C TBM,B mm
0.6
0
21 1.3 0
a Calculated values according to eqs 1 and 2. Produced in the tube under 16 kg/cm2 of pressure and measured in the temperature range from room temperature to 500 'C. 'Shrinkage. dQuality grading as a lump needle coke: R, regular; G, good; E, excellent. 'Carbonization conditions: LSVR/DO = 1/1; temp, 480 O C ; pressure, 8 kg/cm2. f Middle parts of lump cokes free from bottom mosaic were measured from room temperature to 500 "C. #Thickness of bottom mosaic coke. hAreas of flow texture were observed in the bottom region. ' A few spots of mosaic texture were included in the bottom.
saturate, light aromatic (LA), heavy aromatic (HA), and resin (R)fractions by column chromatography using hexane, hexane/benzene (1/7 volume ratio), benzene, and ethanol/benzene (1/9 volume ratio) as solvents, respectively. Decant oils were all soluble in hexane, being free from the asphaltene. The fractions were analyzed with 'H NMR, IR, and gas chromatography. LSVR was fractionated by the solvent extraction and successive column chromatography.
Results Cokes from FCC-DOs in Their Single Carbonization. Figure 1 illustrates montage micrographs of cokes produced from a series of FCC-DOs in a tube bomb at 500 "C under atmospheric pressure. FCC-DOs produced flakes of coke of variable shape and optical anisotropy at various yields. Some DOs (Al, A2, B, C) provided rather large flaky cokes where flow texture was observable, while some others did very small ones of mosaic texture. According to the size of anisotropy, FCC-DOs were graded as follows:
C > D > B > A2 > G > A1 > F > E Figure 2 illustrates montage micrographs of cokes produced at 500 "C under 16 kg/cm2. The solid lump cokes were obtained from all FCC-DOs, although their textures and orientation of optical anisotropy varied significantly from a coke to another. Eight cokes were graded into three classes (excellent, good, and regular) according to the an-
isotropic development, although all cokes exhibited flow texture. The anisotropic development was evaluated in terms of size and shape of anisotropic units, degree of their uniaxial orientation, and homogeneity of their distribution. The grades of cokes are summarized in Table 11. Cokes from E, F, and C oils which were graded as excellent exhibited excellent flow textures of uniformity, oriented almost perfectly along the axis. A few more areas of disordered orientation were found in C and F cokes. B and D cokes were graded as good. Significant areas of mosaic texture, which dispersed in the cokes to deteriorate coke quality, graded them in the order. Although Al, A2, and G cokes were certainly classified as needle cokes, the anisotropic units were rather limited in their size and degree of orientation. Mosaic textures or winded flow textures were found in considerable areas. Such an order of oil grading is definitely different from that of the cokes produced under atmospheric pressure. Cocarbonization with LSVR. Micrographs of lump cokes cocarbonized at 480 "C from LSVR with a series of FCC-D0s are illustrated in Figures 3 and 4, where the cokes were compared at their middle and bottom portions in terms of uniaxial alignment of flow textures and thickness of fine mosaic texture, respectively. FCC-DOs Al, B, E, F, and G gave long-flow textures aligned almost uniaxially. Excellent alignment of narrow stripes was noted in G and B cokes. A2, D, and C certainly did have flow textures; however, their units were rather short and winded, as shown in Figure 3. Hence, all lump
Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 1795
Figure 1. Microphotographs of coke lumps from FCC-DOs. Letters designate the DOs. Carbonization conditions: temperature, 500 "C; pressure, 1 kg/cm2; time, 4 h.
cokes of the present study may be classified as needle cokes; however, their grading can be in the following order: G > B > A1 E F > D A2 > C DOs of A2, E, F, and G gave definite mosaic texture a t the bottoms of their cocarbonized coke lumps as shown in Figure 4, although their thickness and the size of the mosaic units were different. The thickness of their bottom mosaic belts is summarized in Table 11. The bottom of the coke from A1 carried some areas of flow texture in the thin mosaic belt. Oil D produced very small units of anisotropy a t the bottom; however, they were almost aligned uniaxially. The units of flow texture in the coke from oil C were certainly shorter in comparison with those of the middle part, although they were arranged almost uniformly. No mosaic texture was observed in the coke from oil B. Thus, the oils are graded in the following order as the cocarbonization partner in terms of digesting ability against the bottom mosaic: B -C>D>Al>E -F-G>A2 Yields and CTE Values of Lump Cokes. 1. Single Carbonization of FCC-DOs. The yields of coke in the single carbonization of DOs under atmospheric pressure ranged from 3% to 37%. The cokes were flaky; hence their
- -
-
CTE Values were not measurable. The coke yields under 16 kg/cm2 are summarized in Table 11. A marked increase of the yield was observable under the pressure from all oils, although the levels of the yields distributed from 32% to 76%. The major portion of oils which evolved out under atmospheric pressure should stay under 16 kg/cm2 in the tube to be carbonized and influence the quality of the coke. CTE values of lump cokes produced from the FCC-DOs under 16 kg/cm2 are summarized in Table 11. The values which distributed from -0.15 X lo* (shrinkage) of E coke to 0.61 x lo* OC-l of A1 coke indicated that the cokes from all oils were classified as needle cokes of rather low CTE. Nevertheless, the quality of the oils as the feedstocks for the needle coke was indicated nontrivially, a t the same time, to be diverse. 2. Cocarbonization. Table I1 summarizes the coke yields from LSVR and a series of FCC-DOs in the cocarbonization a t 480 "C under 8 kg/cm2. The yields were much the same except for Al, ranging from 20% to 29%. The CTE values of the cokes ranged from 0.2 X lo* (G oil) to 1.3 X lo* OC-l (C oil), reflecting approximately the uniaxial alignment of flow texture. The present conditions may be not necessarily optimum to blends of some DOs.
1796 Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990
I
1 mm
. 1 mm
Figure 2. Microphotographs of coke lumps from FCC-DOs. Carbonization conditions: temperature, 500 "C;pressure, 16 kg/cm2 N2;time, 4 h.
C and D oils which produced no or little bottom mosaic gave large CTE values, while B oil gave an excellent CTE without bottom mosaic. General Properties of FCC-DOs and LSVR. The microanalyses of DOs are summarized in Table I. All DOs carried high carbon content around 90%, while hydrogen and sulfur contents varied significantly from one oil to another, ranging from 7.8% to 10.4% and from 0.1% to 1.4%, respectively. Nitrogen and oxygen contents also varied considerably with oils, although their contents were both very low (less than 0.5%). Proton distributions in whole decant oils are summarized in Table I with their fa (aromaticity) values. The 'H NMR characteristics of decant oils were certainly different from one to another; however, their difference was too limited to differentiate them by fa from 0.55 to 0.69 or proton distribution. Decant oils were revealed by TLC-FID to consist of the saturate, aromatic, and resin fractions, all free from the asphaltene. The aromatic fraction was the major fraction in all decant oils, their contents varying from 60% to 95%, which appear to reflect the fa values. Some oils (B and C) carried a very small amount of the saturate fraction, less than 870,while the other oils carried from 17% to 31% . The resin fraction was always very minor, its content being less than 2%. LSVR consisted of major saturate, aromatic, resin, and asphaltene fractions as summarized in Table I. Composition of broad variety is characteristic to LSVR. The major contribution of asphaltene is definite. Structural Characteristicsof Respective Fractions in DOs. The yields of fractions separated according to
their solubilitieswith a preparative column are summarized in Table I. Hx, Hx/Bz, Bz, and EtOH/Bz fractions may correspond to saturate (St), light aromatic (LA), heavy aromatic (HA), and resin fractions (R) in TLC. The content of Hx (saturate) ranged from 4% (B) to 39% (E). The oils can be classified into three groups according to their contents: group 1 (4-8%): B, C group 2 (23-27%): Al, D group 3 (30-39%): A2, F, G, E The major component of the Hx fraction was found by GC, straight paraffins, the chain length of which ranged from 20 to 30 carbon atoms, regardless of decant oils, as shown by some representative chromatograms in Figure 5. Nevertheless, B, C, and F oils carried a considerable amount of branched paraffins as indicated by the broad background peak in chromatograms of Figure 5. Their amounts varied considerably with oils, A1 and A2 carrying much less. The principal components in the decant oils were light and heavy aromatic fractions, sums of the contents being a t least 60% (E oil) and up to 95% (B oil). Their major contribution to the coke was definite. The ratio of light and heavy aromatic fractions varied from one oil to another: the light fraction was major in Al, A2, C, F, and D oils; similar amounts were found in B and E oils; and the heavy one was major in G. Structural characteristics of light and heavy aromatic fractions are summarized in Tables I11 and IV,respectively. Both fractions were fairly aromatic, the values of fa ex-
Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 1797
+..--I
lmm
Figure 3. Microphotographs of middle portions in the cocarbonized lump cokes from LSVR with FCC-DOs. Carbonization conditions: LSVR/DO, 1; temperature, 480 "C; pressure, 8 kg/cm2. Table 111. Properties of Light Aromatic Fractions A1
A2
B
C
D
E
F
G
90.68 7.33 0.00 1.18 0.81 1.03 34 40 18 8 0.69 0.40 0.43 3.31 198
91.69 7.17 0.00 0.43 0.71 1.07 37 39 16 8 0.72 0.36 0.39 3.43 212
90.26 7.09 0.01 0.84 1.80 1.06 36 40 17 7 0.71 0.39 0.45 2.92 195
90.43 7.12 0.03 1.35 1.02 1.06 41 41 13 5 0.73 0.37 0.26 2.52 162
91.25 7.41 0.00 0.65 0.69 1.03 38 38 16 8 0.71 0.35 0.22 3.38 183
90.97 8.40 0.00 0.07 0.56 0.90 26 33 28 13 0.61 0.40 0.25 6.03 231
89.99 8.79 0.01
89.23 9.09 0.01
0.85 20 37 27 16 0.56 0.50 0.37 6.00 217
0.82 17 31 33 19 0.53 0.50 0.31 8.36 266
Proton chemical shift range: Ha, 10-6 ppm; H,, 4-2 ppm; H,, 2-1.1 ppm; H,, 1.1-0.3 ppm. bAromaticity. Number of alkyl side chains in structural unit according to Brown-Ladner analyses. Fraction of naphthenic ring in structural unit according to Brown-Ladner analysis. eNumber of carbon atoms in alkyl side chains in structural unit according to Brown-Ladner analysis. 'Molecular weight of structural unit according to Brown-Ladner.
ceeding 0.53 (light aromatic) and 0.69 (heavy aromatics), respectively. The structural characteristics of the heavy ones in the oils were rather similar, regardless of the decant oils, in terms of fa, H/C, and proton distribution except for fairly smaller Ha and larger H, with F and G oils. In contrast, those of the light aromatic fractions were rather diverse. According to the indexes, they were classified into two groups: E, F, G and Al, A2 B, D, C. The first group was characterized by lower aromaticity (fa: 0.54.6), smaller C/H (0.&0.9), and more aliphatic protons, especially much larger H, and H, protons and G oil was extreme in the group in terms of Cside, with indications
it could carry very long alkyl side chains. The second group, especially C oil, was more aromatic, carrying fewer alkyl chains. Large Cside values of A l , A2, and D oils were notable among the group, although they were much smaller than those of the first group. Intermediate Products of Cocarbonization. Table V summarizes yields of fractions in the cocarbonization intermediates from B and G oils with LSVR and some of their properties. Both blends, which were completely soluble in benzene before the carbonization, produced a significant amount of BI, around 11% by 1h, still carrying considerableamounts of HS and HI-BS. The intermediate
1798 Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990
Figure 4. Microphotographs of bottom portions in the cocarbonized lump cokes from LSVR with FCC-DOs. For experimental conditions, refer to Figure 3. Table IV. Properties of Heavy Aromatic Fractions
elem. anal., ?% C H N C/H Ha Ha H, H7
fa U
A1
A2
B
C
D
E
F
G
92.10 6.46 0.50 1.19 45 32 19 4 0.77 0.29 0.47 3.22 267
91.92 6.32 0.64 1.21 42 36 19 3 0.77 0.33 0.80 2.73 285
92.23 6.13 0.15 1.25 45 36 17 2 0.78 0.31 0.82 2.53 299
91.64 6.44 0.79 1.19 43 31 22 4 0.77 0.30 0.60 3.30 284
92.13 6.29 0.85 1.22 41 34 22 3 0.76 0.33 0.97 3.12 331
92.86 6.67 0.09 1.16 41 40 14 5 0.75 0.34 0.58 3.02 250
92.24 6.93 0.16 1.11 29 42 20 9 0.69 0.43 1.26 4.23 340
92.16 6.88 0.22 1.12 35 42 17 6 0.72 0.39 0.77 3.16 252
from the blend of LS 'R and oil B which procxed no bottom mosaic texture contained a much smaller amount of HS and more HI-BS. The fa values of both of the fractions which may constitute the matrix were very different, and paraffins were still the major component of the HS fraction, while HI-BS fractions were fairly aromatic. Thus, the aromaticity or dissolving ability of the matrix which may reflect the smaller amount of paraffinic HS fraction is estimated to be very different a t the intermediate stages of cocarbonization. A smaller dissolving ability of the matrix from LSVR and G oil may lead to the precipitation of BI or mesophase, allowing the formation of bottom mosaic texture. Discussion Eight DOs of different origins were proved in the present study to provide needle coke lumps by pressurized single carbonization and cocarbonization in a tube bomb. Nev-
Table V. Fractions of Intermediate Products in the Cocarbonization of DOs B and G with LSVRa fraction B G HS content, % 41 55 C/H/N, 7% 88.96/9.96/0.13 89.50/9.42/0.13 1.30 1.25 H/C fa 0.53 0.55 HI-BS content, % 47 34 C/H/N, % 91.69/6.19/0.25 91.98/6.18/0 0.80 0.80 H/C fa 0.79 0.80 BI content, ?% 12 11 H/C 0.51 0.53 fa a Cocarbonization conditions: temperature, 500 "C;pressure, 8 kg/cm2; time, 4 h.
ertheless, the lumps exhibited variable CTE values and a thickness of bottom mosaic texture, depending upon the
Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990 1799 carbon content of the R fraction, fa(R) the fa value of the R fraction, W(HA) the yield of the heavy aromatic (HA) fraction, fa(HA) the fa value of the HA fraction, and C(HA) the carbon content of the HA fraction. The equation may mean that all aromatic carbon atoms in the resin and the heavy aromatic fractions are carbonized finally into the coke under atmospheric pressure, losing their aliphatic carbons and all hydrogens. The coke yield under 16 kg/cm2 of pressure, Y16,can be correlated by another equation Y16 = W(R)fa(R)C(R)+ W(HA)fa(HA)C(HA)+ W(LA)fa(LA)C(LA) (2)
5
10 15 20 25 ret en t i on time (m in)
Figure 5. Gas chromatographs of paraffin fractions in some typical FCC-DOs. Letters designate the DOs.
natures of oils. Selection of adequate FCC-DO as a partner in the cocarbonization with LSVR is critically important to produce commercially needle lump coke of excellent CTE and uniform quality. It is important to note that the cocarbonization properties of DOs do not always coincide with those in the single carbonization. Excellent E and F oils in their single Carbonization produced bottom mosaic in the cocarbonization. B oil was effective in eliminating the bottom mosaic and provided the smallest CTE in the cocarbonization. C and D oils were also effective in reducing the mosaic but gave larger CTE under the present carbonization conditions. Other DOs produced rather thick mosaic belts at the bottom. On the basis of the present results, correlations between their carbonization properties and structural indexes are of value for some discussion. It may be important to point out that the aromaticity of the FCC-DO is defined by two categories, the content of aromatic fraction in FCC-DO and number of aromatic carbon atoms in the aromatic fraction, because the fraction principally contributes to the carbonization. Alkyl groups may not contribute to the yields of coke but may influence the carbonization. The long alkyl chains on the aromatic ring are eliminated easily at the early stage of carbonization to enrich the saturate and olefin content in the matrix and to reduce the dissolving activity of the matrix. Radical reactions of alkyl chains may also initiate the reaction to accelerate the carbonization. Coke Yield from FCC-DO. The aromatic hydrocarbons in the FCC-DO are thermally condensed into larger aromatic ones of higher boiling point through the radical coupling, otherwise they evolve out. Under pressure, lighter aromatic molecules obtain chances to stay in the bomb until they are condensed. The alkyl side chains may be lost gradually during the carbonization. In contrast, paraffinic hydrocarbons have a very small chance to be thermally aromatized; hence, they hardly contribute to the coke yield. By comparing the fractional composition of oils to their coke yields, the fraction which contributes the coke yield can be defined. The coke yield under atmospheric pressure Yat, can be correlated to the amount of heavy aromatic and resin fractions by Yatm= W(R)fa(R)C(R)+ W(HA)fa(HA)C(HA) (1) where W(R) is the yield of the resin (R) fraction, C(R) the
Light aromatic hydrocarbon (LA) can be carbonized under 16 kg/cm2. Calculated yields (Yatm and Y16) are summarized in Table 11. The values of Yaband Y16 except for Yaw of D, E, F, and G oils coincide fairly well with the observed yields. The assumption that all aromatic carbons in the heavy aromatic fraction are carbonized under atmospheric pressure may not hold with these exceptional oils which exhibited much smaller yields. Some oligomeric molecules consisting of small aromatic rings may evolve out, with no contribution to the yield being expected. A more detailed structure of the heavy aromatic fraction is necessary, for example, in terms of the exact size of the aromatic rings and their connecting bonds. FD-MS and 13CNMR of high resolution may provide further information (Mochida et al., 1990). The present assumption holds very well with the yield under the pressure, where all aromatic rings get more changes to be carbonized. The yield in the cocarbonization under 8 kg/cm2 requires another equation. It is difficult to formularize the interaction with LSVR. CTE of the Coke from FCC-DO. CTE of the cokes along their axis has been related to the anisotropic flow texture in many studies (White, 1976; Marsh and Walker, 1979; Mochida et al., 1987b1, although the contribution of microcracks is definite (Mochida et al., 1976). The present study provided another qualitative correlation as described above. The needle coke has been revealed in both single carbonization and cocarbonization to be produced through two major steps: (1)formation of bulk mesophase of low viscosity through nucleation, growth, and coalescence of mesophase spheres; (2) rearrangement of bulk mesophase into uniaxial orientation by the aid of gas evolution at the solidification of mesophase into the coke (Mochida et al., 1988b,c). A variety of molecules in the oils perform thermal reactions of two major steps under their strong mutual influences. Thus, the idea of cocarbonization among the fractional components even in an oil should be introduced to correlate the structure and reactivity of starting oils to anisotropic structure of the produced cokes. There will be a very small chance to correlate the anisotropic structure with averaged structural indexes of whole starting oils. The fractions which are converted into the coke under 16 kg/cm2 are defined light aromatic, heavy aromatic, and resin fractions as described above. Their chemical structure is of major concern to anisotropic growth. Sufficient aromaticity and naphthenic rings are favorable for the mesophase growth, while long alkyl side chains, oxygen, sulfur functional groups (which tend to accelerate the condensation), and aromatic rings of excess size may hinder the mesophase growth. Hydrogen transfer performed by naphthenic rings is well-documented as being favorable for the development of flow texture (Mochida et al., 1988b) The amount of gas evolution at the solidification stages
1800 Ind. Eng. Chem. Res., Vol. 29, No. 9, 1990
is another factor for the arrangement of mesophase molecules into the needle coke. According to the gas analyses, short alkyl groups which are rather stable on the aromatic rings of mesophase molecules until their solidification can be gas sources (Mochida et al., 1988a) The excellent E and F oils carry considerable amounts of light and heavy aromatic fractions of fairly aromatic natures which may play their respective roles for the mesophase formation in the cocarbonization. They certainly carry a large amount of paraffins, major branched ones of which may be cracked to initiate the pyrolytic reactions of whole paraffins and evolve out not to disturb the mesophase formation. The amount of sufficient alkyl groups of relatively short chain in their light aromatic fraction may be very favorable for the uniaxial arrangement. Another excellent oil, C, is different from E and F. It consists essentially of a light aromatic fraction which is highly aromatic and carries some short alkyl groups. Hence, it is most suitable for the mesophase development, and uniaxial arrangement can be achieved with the least amount of gas evolution. Light aromatic fractions in B and C oils were very similar, although the former oil was essentially free from paraffins while the latter carried 27% of paraffin. A rather limited number of alkyl groups and a considerable amount of heavy aromatic fraction may assure the mesophase formation: however, less gas evolution at the solidification may not be sufficient for the excellent uniaxial rearrangement into the needle coke from B oil. Oils A2, G, and A 1 provided coke of regular grade. A1 and A2 showed very high viscosity at 50 "C (102.8 and 45 cSt, respectively) probably because of a large content of stable long straight paraffins which may cause high viscosity and probably hinder the mesophase development due to their anti-solvent roles, allowing mosaic textures of higher CTE in the resultant coke to give the highest CTE. Thus, the compostion of paraffins in FCC-DO, straight or branched, may influence the carbonization even if they do not contribute to the coke yield because their reactivity for cracking to be evolved out is so different. G oil carried a large amount of heavy aromatics, and their light aromatics carry long alkyl side chains. The high reactivity of the former fraction may not be moderated by the major paraffins and the light aromatics of limited content to give mosaic textures of higher CTE. Lower temperatures (460-480 "C), where the carbonization can be more moderate, provided better CTE as reported in previous papers (Mochida et al., 1988b,c). Cocarbonization Scheme. Interactions among more varieties of fractions in FCC-DO and LSVR define the CTE value and the bottom mosaic quantity of cocarbonized coke. High reactivity of asphaltene in LSVR should be mediated by aromatic fractions in DOs as the cocarbonization partner to give a needle coke of uniform texture. The thermal stability and dissolving ability of the aromatic fractions in DOs can control the carbonization to keep the viscosity of carbonization intermediates low and accelerate the mesophase growth and coalescence. In turn, gas evolution from LSVR may accelerate the uniaxial arrangement into flow texture at the solidification stage. Such roles of partners in the cocarbonization allow the formation of excellent coke (Mochida et al., 1989). Some DOs of low aromaticity and high reactivity which cannot mediate the reactivity of LSVR fail to give better CTE. Oil A2 is such a case. C and D may be too stable in the carbonization a t 480 OC, delaying the solidification with less evolution of gas;
a slightly higher temperature may be more appropriate. Thus, variable properties of D0s may require their respective optimum temperature of the carbonization to produce the best coke as expected from the mechanism described above. High aromaticity of FCC-DO produced a highly aromatic HI-BS fraction as a carbonization intermediate with a smaller amount of paraffinic HS, which can dissolve the mesophase a t an early stage of the carbonization derived from the heaviest and most reactive fraction of the asphaltene in LSVR. In contrast, FCC-DOs rich with paraffins and long alkyl chains (Al, A2, E, F, G ) produce a bottom mosaic in the cocarbonization. The stable straight paraffins tend to work as antisolvents, leading to the phase separation and increased viscosity of the mesophase, limiting the growth of anisotropic units. The mesophase in such a situation should precipitate to the bottom of tube to form a mosaic belt. The qualitative correlation between structure and cocarbonization properties of DOs for the formation of a bottom mosaic suggests that the modification procedures may improve the cocarbonization properties of each FCC-DO, being subject to their respective structure. Dewaxing may be sufficient for some FCC-DOs which carry a large quantity of paraffins. Other DO aromatic fractions which carry long alkyl chains may require dealkylation through thermal and catalytic reactions. The reactive FCC-DO should be modified chemically through hydrogenation and/or depolymerization (Park et al., 1986). Such modification of DOs will be reported later. Such modification can be also effective to improve the CTE of cocarbonized coke. It should be noted that excellent needle coke of low CTE without bottom mosaic coke can be produced by careful optimization of the cocarbonization suitable for the respective blends.
Literature Cited Eser, S.; Jenkins, R. G. Carbonization of petroleum feedstocks I: Relationships between chemical constitution of the feedstocks and mesophase development. Carbon 1989,27, 877-887. Iwata, K.; Itoh, H.; Ouchi, K. Structural Analysis of Quinoline and Pyridine Extracts of Coal. Fuel Process. Technol. 1980,3,25-38. Marsh, H.; Walker, P. L., Jr. Chemistry and Physics of Carbon; Walker, P. L., Jr., Ed.; Marcel Dekker: New York, 1979; Vol. 15, p 228. Mochida, I.; Ogawa, M.; Takeshita, K. Anisotropy of needle cokes. Bull. Chem. Soc. Jpn. 1976, 49, 514. Mochida, I.; Korai, Y.; Nesumi, Y.; Oyama, T. Carbonization in a tube bomb. 1. Carbonization of petroleum residue into a lump of needle coke. Ind. Eng. Chem. Prod. Res. Deu. 1986, 25, 198-201. Mochida, I.; Korai, Y.; Fujitsu, H.; Oyama, T.; Nesumi, Y. Evaluation of several petroleum residues as the needle coke feedstock using a tube bomb. Carbon 1987a, 25, 259-264. Mochida, I.; Korai, Y.; Oyama, T. Semi-quantitative correlation between optical anisotropy and CTE of needle coke grains. Carbon 198713, 25, 273-278. Mochida, I.; Oyama, T.; Korai, Y. Formation scheme of needle coke from FCC-decant oil. Carbon 1988a, 26, 49-55. Mochida, I.; Oyama, T.; Korai, Y.; Fei, Y. Q. Study of carbonization using a tube bomb evaluation of lump needle coke, carbonization mechanism and optimization. Fuel 1988b, 67, 1171-1181. Mochida, I.; Oyama, T.; Fei, Y. Q.; Furuno, T.; Korai, Y. Optimization of carbonization conditions for needle coke production from a low-sulphur petroleum vacuum residue. J . Mater. Sci. 1988c, 23, 298-304. Mochida, I.; Korai, Y.; Oyama, T.; Nesumi, Y.; Todo, Y. Carbonization in the tube bomb leading to needle coke: 1. Cocarbonization of a petroleum vacuum residue and FCC-decant oil into better needle coke. Carbon 1989,27, 359-365. Mochida, I.; Korai, Y.; Azuma, A.; Kitajima, B. Structure and stabilization reactivity of mesophase pitch derived from FCC-DO. Fuel Sci. Technol. 1990, submitted. Nesumi. Y.; Todo, Y.; Oyama, T.; Mochida, I.; Korai, Y. Carboniza-
I n d . Eng. Chem. Res. 1990,29, 1801-1807 tion in the tube bomb leading to needle coke: 11. Mechanism of cocarbonization of a petroleum vacuum residue and a FCC-decant oil. Carbon 1989,27, 367-373. Nesumi, Y.; Azuma, A.; Oyama, T.; Todo, Y.;Mochida, I.; Korai, Y. Cocarbonization of low sulfur vacuum residue and FCC-decant oil assisted by an additive pitch of high aromaticity. J. Petrol. Inst. Jpn. 1990, submitted. Park, Y. D.; Korai, Y.; Mochida, I. Preparation of anisotropic mesophase pitch by carbonization under vacuum. J. Mater. Sci. 1986,21,424-428.
Romero, E.; Menendez, R.; Marsh, H.; Reinoso, F. R. Structual
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studys of cokes from petroleum mixed feedstocks. Carbon 88; Newcastle upon Tyne, UK, 1988, Abstract p 256. Stocks, C. A.; Guercio, V. J. Feedstocks for carbon black, needle coke and electrode pitch. Erdol Kohle Erdgas Petrochem. 1985,38,31. White, J. L. Petroleum Derived Carbon. Mesophase mechnisums in the formation of the micro structure of petroleum coke. ACS Symp. Ser. 1976,21, 282. Received for review October 24, 1989 Revised manuscript received April 23, 1990 Accepted May 14, 1990
Role of Sulfur in Catalytic Reforming of Hydrocarbons on
Platinum-Rhenium/Alumina Garry M. Bickle,’ Jorge N. Beltramini, and Duong D. Do* Department of Chemical Engineering, University of Queensland, St. Lucia, Queensland, 4067 Australia
Cyclohexane reforming and n-heptane reforming were studied on 0.3-0.390 Pt-Re/Al2O3-0.95% C1 catalyst in the presence of sulfur. The combined effect of Re-S and Pt-Si (Siis the irreversible form of platinum) that modified the platinum ensemble size and the electronic interactions caused the hydrogenolysis and dehydrocyclization activities t o decrease. The presence of reversible sulfur (S,) is explained in terms of the equilibrium between hydrogen sulfide and adsorbed S,. Due to the added effect of Re-S decreasing the ensemble size, the fewer “available” free Pt crystallites on Pt-Re/A1203 compared to Pt/A1203means the toxicity of S, was greater for the bimetallic catalysts. The model previously proposed for sulfided Pt/A1203can be extended to Pt-Re/A1203 after taking into consideration the additional effect of ensembling due to Re-S. Spillover of adsorbed hydrocarbon species to alumina during periods of high S, levels has been proposed for Pt-Re/A1203.
Introduction Pt/A1203has been used as a bifunctional reforming catalyst in the oil industry since the 1950s. Despite its good performance, refiners have been examining ways of improving reformer performance by either improving the octane number of the product or lengthening the catalyst’s life. Kluksdahl (1968) patented the addition of rhenium metal to Pt/A1203,producing the first bimetallic catalyst. Later, McCallister and O’Neal (1971), Garten and Sinfelt (1980), and Volter et al. (1981) showed that germanium, iridium, or tin could be added to Pt/A1203to also produce successful bimetallic reforming catalysts. The manner in which each of the bimetallic catalysts improves performance varies. Sinfelt (1984) has indicated that the iridium species increases the coke hydrogenation. Biloen et al. (1980) showed that rhenium selectively hydrogenolyzed coke precursors while De Jongste and Ponec (1980) showed that tin decreased the ensemble size of the platinum surface. Despite knowing the manner in which each of the bimetallic reforming catalysts improved the performance, the state of the second metal was still in debate. Burch (1981),Burch and Mitchell (1983), and Burch et al. (1983) found that tin can exist both as an alloy with platinum and as separate Sn2+ions on the alumina surface, segregated from the platinum. Biloen et al. (1980), Coq and Figueras (1984),Coughlin et al. (1984),Volter and Kurschner (1983), Lietz et al. (1984),and Lieske et al. (1987) have shown that a Pt-Sn alloy alone exists. The presence of tin decreases the ensemble size such that the geometric effect reduces
* Author for correspondence. ‘Present address: Laboratorium vmr Petrochemisch Techniek Rijksuniversiteit Gent, 9000 Gent, Belgium. 0888-5885/90/2629-1801$02.50/0
metal site coke formation. Lieske et al. (1987) hypothesized that the presence of tin caused the coke precursors, which couldn’t further react on the metal due to the geometric effect, to “drain off“ to the alumina surface, producing acid site coke. For the Pt-Re/Al,03 system, Johnson and LeRoy (1974), Bertolacini and Pellet (1980), Kelley et al. (1982), and Mieville (1984) have observed separate metal clusters of platinum and rhenium (as Re4+). On the other hand, the existance of Pt-Re alloy has been shown conclusively by Biloen et al. (1980), Bolivar et al. (1976), Issac and Petersen (1982), Davis et al. (1982), Sachtler (1984), Apesteguia and Barbier (1982), Menon and Froment (1984), and Jothimurugesan et al. (1985a,b). To further confuse the situation, the presence of water in the system affects the degree of hydration of the rhenium oxide and hence the state to which it can finally be reduced as stated by Webb (1975), Johnson (1975), McNicol (1977), and Charcosset et al. (1979). Schay et al. (1984) was able to show that alloy and segregated clusters of platinum and rhenium could co-exist under various conditions of regeneration. The last two points show that the state of the second metal is a function of the pretreatment conditions (temperature and time), which are critical to ensure complete alloying and reduction of both platinum and the second metal. Furthermore, Van Trimpont et al. (19851, Shum et al. (1985), Barbier and Marecot (1982), and Bergeret and Gallezot (1984) found that Re/A1203is a good catalyst for hydrogenolysis. Recently, Jothimurugesan et al. (1985a,b) showed that Re/A1203did not have any hydrogenolysis activity below 425 O C . Kubicka and Okal (1987) further showed that benzene hydrogenation did not occur and hypothesized that, at low rhenium concentrations or high 0 1990 American Chemical Society
