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Ind. Eng. Chem. Prod. Res. Dev. 1082, 21, 315-320
Catalytic Carbonization of Vacuum Residual Oil from Orinoco Tar Sand Isao Mochlda, Yoshlakl Takeshlta, Yoro Koral, Hlroshl Fujltsu, and Kenjlro Takeshlta Research Instltute of Industrial Science, Kyushu University 86, Kasuga 8 16, Japan
Orinoco vacuum residue of a heavily contaminated oil was carbonized in the presence of aluminum chloride under variable heating rates to reveal its influence on the quality and quantity of gas, oil, and coke products. The catalyst accelerated the desulfurization and denitrogenatlon reactions to produce the coke and oil of higher purity. The catalyst could modify the coking properties of the residual oil to produce flow texture. The coke and gas yields were in general increased by the catalyst; however, the higher heating rate (1200 OC/h) could produce a considerable amount of clean oil. Addition of hydrogen donating additive such as 9,lOdlhydroanthracene promoted the desulfurization and mesophase development, consequently reducing the catalyst amount required for the sufficient desulfurization and flow texture. Naphthenic intermediate which, being itself fusible, can dissolve the heavier products by hydrogen transfer may play an important role In these processes. Hydrogen donating addiiie may play the same role.
Introduction Effective utilization of petroleum heavy residual oils containing so much sulfur and nitrogen as well as metals has become an important task in the present petroleum industry since the crudes tend to be heavier and severely contaminated (Miura, 1981). Hydrodesulfurization or hydrotreatment and catalytic cracking may be the most extensively studied methods for upgrading of these oils (Bridge et al., 1976);however, there are some disadvantages such as the short life of the catalyst due to the deposit of carbonaceous material and high cost of hydrogen consumed (Turnock, 1976). Although the coking process can be another effective procedure to produce clean liquid fuel from such dirty o h , a significant amount of the cokes produced simultaneously contain much sulfur (Speight, 1970) and metals (Thomas et al., 1979) and/or often fail to give a favorable structure (texture) (Kipling and Shooter, 1966), limiting their value. The present authors have reported favorable effects of aluminum chloride on the desulfurization (Mochida et al., 1977/1978) and structural modification of cokes produced in the coking process of the residual oils (Mochida et al., 1976a; Mochida and Takeshita, 1977a). In the present study, the catalytic carbonization of Orinoco vacuum residue was investigated using aluminum chloride to reveal its effects on the quality of carbonization products, gas, distilled oil, and coke. The effects of the hydrogen donative cocarbonization additive, 9,lO-dihydroanthracene in the present case, were examined in the catalytic carbonization since it has been reported to promote desulfurization from cyclic sulfur compounds and to help the development of anisotropic flow texture from various organic substances including pitches, coals, and heterocyclic compounds (Mochida et al., 1979a, 1980). Such carbonization may be able to produce valuable cokes as well as clean fuel fluid, making the coking process economically feasible for upgrading the heavy residual oils (Mochida, 1980). Experimental Section 1. Materials. Vacuum residue of Orinoco tar sand (Orinoco VR) was supplied by Chiyoda Chemical Construction Co. Japan, and some of its properties are listed in Table I. Aluminum chloride (Kishida Chem. Co., Japan) and 9,lGdihydroanthracene (Tokyo Kasei Co., Japan) 0198-4321/82/1221-0315$01.25/0
Table I. Properties of Orinoco Vacuum Residue Orinoco sp. gr. 15/4"C CCR. wt % C
1.0197 16.4 84.10 10.34 4.05 0.67 457 107 130 58 0.317 32 778
H S N
v, PPm Ni Fe Na ash, wt % S.P., "C mol wt
were used without further purification. 2. Carbonization. Orinoco VR (2-4 g) and aluminum chloride (0-30 w t %) mixed in a mortar were heated in a Pyrex tube (300 X 30 mm) to 600 "C and held for 2 h at the temperature under nitrogen gas flow. The rate of temperature rise was programmed a t 50-1200 OC/h. Carbonization products were classified as coke, distilled oil, and gas. Distilled oil was trapped in an ice bath for further analysis. Their yields were defined by the following equations to exclude the remaining A1Cl3 (Mochida et al., 1976a). coke yield =
wC+A X
WP
(CC+A
(C,
+ HC+A) x 100 + H,)
gas yield = 100 - coke yield - oil yield
(1)
(3)
W, = weight of fed pitch; Wc+A = weight of coke containing remaining aluminum chloride; Wo = weight of oil; C = carbon content of fed pitch; CC+A= carbon content ofcoke containing aluminum chloride; C, = carbon content of oil; Hp = hydrogen content of fed pitch; H C + A = hydrogen content of coke containing aluminum chloride; and H, = hydrogen content of oil. 3. Analytical. The optical texture of coke was examined with a Nikkon POH microscope using reflected polarized light under crossed Nicols. Sulfur in the oil was analyzed using a Horiba SLFA 200 X-ray fluorescence 0 1982 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982
aI
Table 11. X-ray Structural Parameters of Graphites (HTT 2600 'C/0.5 h)
0 10 30
6.74 6.73 6.73
260 479 401
Table 111. Analysis of Cokes
A l a , , wt % 0
10
20
30
b
0
C H N S ash N/C S/C
86.38 2.41 2.05 9.17 1.40 2.03 X l o - ' 3.98 x lo-'
10
30
73.63 2.65 1.35 3.67 13.40 a 1.57 X 1.87 x
63.75 2.31 0.82 2.29 15.55" 1.10 X 1.35 x
lo-' lo-'
lo-' lo-'
" Calculated from removed sulfur from pitch (heating rate, 150 "C/h). Contains remaining AlC1,.
0
10 AlCl,
20 30 a m o u n t ( w t '1.)
amount of charged aluminum chloride were varied. In general, the yields of coke and gas increased and that of oil decreased in the catalytic carbonization in comparison with the thermal one, although the extents depended considerably on the amount of the catalyst and the heating rate. The coke yield, which was around 20% without the catalyst, tended to decrease slightly with increasing heating rate, reaching the maximum around 40% by aluminum chloride of 30 wt % The gas yield which was also around 20% without the catalyst, tending to decrease slightly with increasing heating rate, increased sharply by 10% of aluminum chloride, still depending on the heating rate. The lowest heating rate of 50 "C/h gave yields as high as 60% while the highest rate of 1200 "C/h gave only 20%. Further increase of catalyst amounts was not effective. The yield of oil which depended considerably on the heating rate (the slower rate gave the lower yield) decreased to zero by 30 wt % of aluminum chloride at the rate of 50 "C/h; however, it could be 30% when the rate was 1200 "C/h at the same catalyst level. Thus, the addition of catalyst is in general favorable for the production of gas and coke, although some extent of oil is producible. The influence of the catalyst on the quality of the products should also be taken into account. Properties of Coke Produced by Catalytic Carbonization. The optical micrographs of cokes produced from Orinoco VR were shown in Figure 2, where the effects of the catalyst were clearly indicated. Although the Orinoco VR alone produced a fine-mosaiccoke, the catalyst allowed the development of larger anisotropic unit in the cokes. Coarse mosaic or flow texture prevailed in the photographs of the cokes produced with 10 and 30 w t % of aluminum chloride, respectively. X-ray structural parameters (C, (002) and L, (002)) of graphite prepared from the coke a t 2600 "C are summarized in Table 11. The crystalline height was significantly improved by aluminum chloride in the carbonization step. Chemical analyses of the cokes are shown in Table 111. The catalyst significantly decreased nitrogen and sulfur contents in the coke. As reported previously, the catalyst is active to eliminate hydrogen sulfide from sulfur compounds including heterocyclics (Mochida et al., 1979b). Nitrogen in metal porphyrins which are known to be a major form of metal and nitrogen constituents in such a metal-rich residue (Hitchon, 1971) can be removed by their decomposition catalyzed with aluminum chloride during the carbonization reaction.
.
I 0
I
10 ALCla
I
I
20 30 a m o u n t (wt % )
Figure 1. Yields of gas, distilled oil, and coke in the catalytic coking of Orinoco VR under variable heating rates: (a) 50 OC/h; (b)600 O C / h ; (c) 1200 O C / h ; (0) gas yield; (A)oil yield; ( 0 )coke yield.
spectrometer. Average molecular weight of the oil was measured with a vapor pressure osmometer (Hitachi Perkin-Elmer 115) using benzene as the solvent. lH N M R spectra were obtained by a JEOL high-resolution FT-NMR spectrometer, using deuterated chloroform as the solvent. Evolved hydrogen and methane gas were analyzed by a gas chromatograph (TC detector) using a 3.4-m column packed with 13X molecular sieves at room temperature. Evolved C2-C4gases were analyzed by a Varian Aerograph 1400 gas chromatograph (hydrogen flame detector) using a 5-m column packed with vz-7 at 30 "C. Hydrogen sulfide evolved which was trapped in aqueous Cd(CH3C00)2,was quantified according to the Japanese Industrial Standards K2302. The lattice constant, Co (002), and average crystalline height, L, (002), of the graphites prepared from the coke were determined from the (002) diffraction peak according to the standard method prescribed by Japan Society for the Promotion of Science (1963). Results Yields of Coke, Distilled Oil, and Gas from Orinoco VR in Its Catalytic Carbonization. Yields of coke, oil, and gas from Orinoco VR in its catalytic carbonization are illustrated in Figure 1, where the heating rate and the
Ind. Eng. Chem. Prod. Res. Dev.. Vol. 21, NO. 2, 1982 317
d
Figure 2. Optical micrographs of cokes from Orinoco VR: H I T , 600 'C, 2 h; heating rate. 600 'C/h; (a) Orinoco V R (b) Orinoco VR + AICI, 10 w t %; (e) Orinoco VR + AICI, 30 w t %; (d) dihydroanthracene + AICI, 10 wt %; (e) Orinoco VR + dihydroanthracene 20 wt %
+ AICI, 10 wt %.
Table IV. Properties of Oils
yield, wt % color
viscosity mol w t
AICI,, wt % Orinoco, VR 0 10 30 ~. . 70.7 61.2 32.1 black brown brown colorless high low low low
280 450 C/H 0.68 0.61 fa 0.315 0.243 sulfur, wt % 4.05 2.41 nitrogen, wt 90 0.67 0.50
temp," "C
778
-
303 350 0.62 0.264 1.30 0.31
303 200 0.50 0,009 0.02 0.15
a Temperature a t which oil evolution begins (heating rate, 1200 'C/h).
The ash content, which may include the remaining aluminum chloride, was high in the catalytic carbonization. When the additional ash content in the coke is assumed to be alumina from the catalyst, a major portion of the catalyst (100%and 79% of the added catalyst at the addition of 10 and 30 wt %, respectively) remains in the coke. Extensive washing with dilute hydrogen chloride can remove the catalyst in the coke, although the native ash component may remain in the coke. Properties of Distilled Oil. Some properties and NMR spectra of distilled oils are shown in Table IV and Figure 3, respectively. The yield of most wanted distilled oil decreased by the larger amount of aluminum chloride as described above; however, the catalyst reduced significantly its color, viscosity, C/H ratio, and nitrogen and sulfur contents.
Figure 3. 'H NMR spectra of Orinoco VR (a) and distilled oil produced in the coking process (b)-(D: (a) Orinoco V R (b) Orinoco
+ AlCl, (0wt %); (e) Orinoco VR + AICI, (IO wt %); (d) Orinoco VR + AICI, (30 wt %); (e) Orinoco VR + dihydroanthracene (20 w t %) + AIC4 (IO wt %); (Ddihydroanthracene + AICI, (IO wt %). VR
The NMR spectra shown in Figure 3 indicate that the distilled is oil composed principally of aliphatic hydrocarbons, the product with 30 wt % aluminum chloride
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982
c
FO 5
x 04 0
-e 0 3 W
02 01
0 200 300 403 Mo
m -
Temperature ( 'C
1
I 0
J
f"'; P 3
100 200
'
I
E
I
1 300 400 500 600 Temperature ( 'c
I
soak Temperature
(TI
Figure 4. Evolved gases during the coking of Orinoco VR (A) Hz; (B)C1 gas; (C) C2gas; (D) C3gas; (E) C4gas; (0) AlCl, AlCl, (10 w t %); ( 0 )AlCl, (30 wt %); HTT,600 "C, 2 h; heating rate, 150 "C/h.
(0 wt %); (A)
Table V. Analyses of Evolved Gases (Heating Rate, 150 "C/h,600 "C/2h ) _ _ _ _ _---_ -
AlCl,, wt %
.--
gas, wt %
0
30
H*
2.0 35.7 14.6 22.1 25.5
1.2 3.9 8.9 23.0 63.1
c, c, c 2
C,
being free from aromatics. The content of methyl group increased with increasing amount of the catalyst, indicating the increasing extent of branching in the alkyl chains. Thus, the catalyst tends to decrease the yield of oil; however, it can improve the quality of the distilled oil. Evolved Gas. Figure 4 shows the evolution profiles of H, and C1-C4 gases at the heating rate of 150 "C/h. Aluminum chloride enhanced the evolution of these gases except for methane. The temperature of the maximum evolution of hydrocarbon except for methane was shifted from 500 "C to 300 "C, whereas those for H2and methane were not influenced as much. The concentrations (wt %) of these gases in the evolved gas were summarized in Table V. Methane was the major product with a considerable amount of C4and C3gases in the carbonization without the catalyst; however, C4 gas became dominant by far in the catalytic carbonization (A1C13, 30 wt %). A considerable amount of the fraction which may be distilled out without the catalyst may be cracked into C4 gases. Desulfurization during the Carbonization. The profiles of hydrogen sulfide evolution during the carbon-
601
Figure 5. Desulfurization profile during the coking of Orinoco VR:
+
(0) 1200 "C/h; (A) 600 "C/h; (0) 50 OC/h; (A) 600 OC/h MC13 (10 wt %); (0) 600 "C/h DHA (20 wt %) AlCl, (10 wt %); (m) 1200 OC/h + AICI, (30wt %); (A)600 "C/h + AlCl, (30wt %); ( 0 ) 50 OC/h AlCls (30 wt %).
+
+
+
Table VI. Desulfurization Ratio heating rate, "C/h
Aw,
50
150
600
1200
0 5
36.2
22.3
20.5
10
62.9 70.3
29.5 35.4 54.3 64.7
56.2 68.0
53.3 62.5
wt%
30
ization are illustrated in Figure 5, where the amount of aluminum chloride and the heating rate were variable. The evolution of the gas, of which starting temperatures were
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982 319 Table VII. Effect of Hydrogen Donative Solvent on Coking desult
coke, S %
oil, S %
gas
cocarbonization of VR and DHA
68.7
35.7 (1.34)
53.6 (1.13)
10.7
F
corrected value of above run
68.7
single carbonization of DHA single carbonization of VR
54.3
a Catalvst concentration. 10%: heating rate. 600 "Clh. was carbonized similarly in its single carbonization.
coke text
29.4 (2.04)
61.5 (1.24)
9.1
F
61.0
22.0
17.0
F
34.8 (3.79)
43.9 (1.20)
21.3
Mm
Correction was made by assuming that the additive of 20 wt %
lowered with its reduced cumulative extent at the higher heating rate, was greatly enhanced by the catalyst (Yamaguchi et al., 1973; Yamaguchi et al., 1975), the slower heating rate being fairly favorable as shown in Table VI, where the cumulative desulfurization extents are summarized. As high as 70% sulfur in the original VR was removed as H2S during the catalytic carbonization using 30% aluminum chloride. The desulfurization which took place principally above 400 "C without the catalyst was quite vigorous around 200 O C with the catalyst. Effects of Hydrogen Donative Additive on the Catalytic Carbonization. The yields of coke, distilled oil, and gas and their sulfur contents in the catalytic carbonization of Orinoco VR in the presence of 9,lO-dihydroanthracene (20 wt %) are summarized in Table VII, where the carbonization properties of the additive are included to estimate its contribution to the products in the cocarbonization. If the additive in the VR (20 w t %) was carbonized similarly in its single carbonization, the product yield from the VR should be corrected as shown in column 3 of Table VI. In comparison with the single carbonization of VR, the cocarbonization decreased the yields of coke and gas considerably to increase the yield of distilled oil. Figure 3e and f a r e the NMR spectra of the oils catalytically produced in the cocarbonization and the single carbonization of 9,10-dihydroanthracene, respectively. Spectrum f indicates a major amount of tetraline with little dihydroanthracene and anthracene in the distilled oil from 9,lO-dihydroanthracene. Their amounts were further quantified by gas chromatograph. Spectrum e indicates that the principal components of the oil are aliphatic, ruling out a significant contribution from reacted dihydroanthracene to increase the yield of distilled oil in the cocarbonization. The cocarbonization with 10% aluminum chloride produced a coke of flow texture which required 30 wt % of the catalyst without the additive as shown in Figure 2. The desulfurization with 10% aluminum chloride was also enhanced by the additive to the same extent achieved by 30 wt % of the catalyst. Although the desulfurization with the additive started at the same temperature to that in the single carbonization, it continued to a higher temperature where the reaction should finish without the additive as shown in Figure 5. Thus, the additive can behave as if it catalyzed the desulfurization and the development of flow texture, reducing the yields of coke and gas.
Discussion Roles of Aluminum Chloride in the Catalytic Carbonization. Aluminum chloride, a typical strong Lewis acid, can promote against complex molecules of VR the condensation, dealkylation, cracking, and desulfurization reactions, all of which influence the yield and quality of the products in the carbonization. The VR consists principally of aromatic and aliphatic components, the latter being further classified into alkyl and alicyclic groups as shown in Figure 6. Products in the thermal and catalytic coking are related to the constituents of the VR in
catalytic
Figure 6. Product balance in the coking. Table VIII. Structural Indicesa of Orinoco VR Orinoco VR 0.31 3 0.732 0.624 0.342 12.9 41.2 28.3 9.45 2.72 60.2 4.68 1.95 22.4 592 778 a
See literature (Iwata e t al., 1980).
Figure 6. The alkyl chains attached to the aromatic nucleus may be dealkylated and cracked into the distillate as postulated by Solomon and Miknis (1980), deep cracking into the short chains less than five methylene units increasing the gas yield. Some of aromatic components can be included in the oil without the catalyst, the fa value of the oil being consistent with that calculated based on the yield; however, they can be effectively condensed to coke by the catalyst as indicated by the carbonization of aromatic hydrocarbons such as naphthalene and anthracene (Mochida et al., 1975a, 1977a). The condensation reaction may take place so effectively that essentially no aromatic hydrocarbon is included in the oil and gas as shown in Table IV when a sufficient amount of the catalyst is charged. The maximum coke yield of 40% in the catalytic carbonization is slightly greater than the fa value of 31% (aromatic carbon/total carbon X 100) which is the percentage of aromatic carbon in the VR. According to the structural analysis (Brown and Ladner, 1960; Table VIII), alicyclic carbons of 14% (alicycliccarbon/total carbon) were included in the residue. Significant portions of the alicyclic components, especially when linked to the aromatic nuclei, can be thermally dehydrogenated to aromatic hydrocarbons during the carbonization, the
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982
latter being further catalytically carbonized to increase the coke yield. Otherwise, they are cracked into the distillate as postulated by Solomon (1981). The catalyst has been reported to catalyze the condensation of aromatic nuclei through the protonated carbonium ion, leaving some naphthenic intermediates for a certain period during the carbonization. Such intermediates are very favorable for the development of a broader anisotropic unit in the resultant coke through the mesophase mechanism as previously discussed (Mochida et al., 1975b, 1976b; Mochida and Takeshita, 197713). The sulfur contents of distilled oil and coke catalytically produced in the present study may be sufficiently low for the commercial purpose. The mechanism of desulfurization catalyzed by aluminum chloride has been discussed using model heterocyclic sulfur compounds (Mochida et al., 1979b). The mechanism may be related to the condensation or coke formation of heterocyclic components of the VR,the naphthenic intermediates playing important roles in the evolution of hydrogen sulfide (Mochida et al., 197711978). The alkyl side chains, which are the principal source for the distillate as described above, contain the least sulfur atoms or contain them in the forms of thiol and thioether types. Both types are easily eliminated by the acidic catalyst. The influence of the heating rate on the desulfurization extent and the oil yield (in relation to the gas yield) may be worthy of some discussion. The evolution of both hydrogen sulfide and C3-C4 hydrocarbons took place predominantly at the temperature range between 200 and 350 "C where the distillation and the chemical relation may compete. The residence time at this range may influence both yields. Thus, the high heating rate leading to the shorter residence time to allow the cracking and desulfurization reactions at these temperatures results in lower evolution of hydrogen sulfide and hydrocarbon gases (higher oil yield). Roles of Hydrogen-Donating Additive in the Catalytic Carbonization, Favorable influence of hydrogendonating additive has been reported by the present authors for the development of anisotropic texture in various cocarbonization processes (Mochida et al., 1979b, 1980). It is proved to be true in the carbonization of the VR by the fact that the additive could reduce the catalyst amount required for the development of the needle-like texture. Roles of the hydrogen-donating additive are to dissolve and/or hydrogenate through hydrogen transfer heavily condensed intermediates of the carbonization as discussed in previous papers (Mochida et al., 1979b, 1980). Such situations may allow the carbonization system to keep the liquid phase of low viscosity until the later stages, being favorable for the growth of mesophase into the needle-like texture (Mochida et al., 1977b). The favorable influence of the additive on the desulfurization can be explained by the same factors as for the partial hydrogenation of the reaction phase since the desulfurization requires the hydrogenation around the sulfur
moieties in the heterocycles before the solidification, where their contact with the catalyst is not efficient any more. The sulfur atom in the coke may require a much higher temperature to be eliminated thermally. Such an explanation is supported by the fact shown in Figure 5, that the additive allows the desulfurization at higher temperature where the system should be a solid phase without the additive. Although rather contradictory effects of the hydrogen donating additive have been reported in the various types of catalytic desulfurization (Kikkawa et al., 1980), its mechanism looks sound and consistent in the acidic form using aluminum chloride for the heterocyclic sulfur and VR. Efficient hydrogen transfer from the additive may help the alicyclic component to distil out as the oil during the catalytic carbonization, decreasing the coke yield. The recovery of the catalyst is one of the largest difficulties in the catalytic carbonization (Mochida, 1980), although the process carries various advantages. The most desirable solution is to minimize the amount of the catalyst to dispense with its recovery. The use of the proper additive at the selected temperature can be a guideline to achieve the requirement. Increased oil yield by the additive can be another benefit in the catalytic cocarbonization. Literature Cited Bridge, A. G.; Reed, E. M.; Scott, J. W. J . Jpn. Pet. Inst. 1978, 19, 96. Brown, J. K.; Ladner, W. R. Foel 1980, 3 9 , 79. Hltchon, B. "Origin and Refining of Petroleum"; American Chemical Society, Washington, DC, 1971; Chapter 2. Iwata, K.; Itoh, H.; Ouchi, K. FUelPmcess. Techno/. 1980, 3 , 25. Japan Society for the Promotion of Science, Tanso 1983, 3 6 , 25. Kikkawa, S.; Nomura, M.; Mukaihara, F. J . Jpn. Pet. Inst. 1980, 2 3 , 390. Kipling, J. J.; Shooter, P. V. Carbon 1988, 4 , 1. Miura, E. Chem. Chem. Ind. 1981, 3 4 , 77. Mochida, 1. Shokubal1980,2 2 , 88. Mochida, I.; Amamoto, K.; Maeda, K.; Takeshita, K. J. Jpn. Pet. Inst. 1977b, 2 0 , 1027. Mochida, I.; Ando, T.; Maeda, K.; Fujitsu, H.; Takeshita, K. Carbon 1980, 18, 319. Mochida, I.; Ando, T.; Takeshita, K. J . Fuel Soc. Jpn. 1979b, 58, 321. Mochida, I.; Kaji, N.; Hayama, Y.; Maeda, K.; Takeshita, K.; J . Jpn. Pet. Inst. 1978a, 79, 9. Mochida, I.; Kudo, K.; Fukuda, N.; Takeshita, K.; Cerbon 1975a, 73, 135. MochMa, I.; Maeda, K.; Takeshita, K. High Temp. High Pressures 19778, 9 , 123. Mochida, I.; Maeda, K.; Takeshita, K.; Keji, N.; Suetsugu, Y.; Yoshda, T. Fuel Process. Techno/. 197711978, 1 , 103. Mochida, I.; Moriyama, S.; Matsuoka. H.; Maeda, K.; Fujitsu, H.; Takeshita, K.; Marsh, H. J . Fuel Soc. Jpn. 1979a, 56, 848. Mochida, 1.; Nakamura, E.; Maeda, K.; Takeshita, K. Carbon 1975b, 73,489. Mochida, 1.; Nekamwa, E.; Maeda, K.; Takeshita, K. Carbon 1978b, 14, 123. MochMa, I.; Takeshita, K. J . Jpn. Pet. Inst. 1977a. 2 0 , 3. Mochida, I.; Takeshita, K. J . Jpn. Pet. Inst. 1977b, 2 0 , 183. Solomon, P. R. Fue/ 1981. 6 0 , 3. Solomon, P. R.; Mknis, F. P. Fuel 1980, 56, 893. Speight, J. G. Fue/ 1970, 49, 134. Thomas, R. J.; Ernest, A. S.; James, E. 2 . Fuel 1979, 58. 589. Turnock, P. H. J. Jpn. Pet. Inst. 1978, 79, 146. Yamaguchi, T.; Goto, T.; Kamiguchl, T.; Okita, S.; Ito, T. J. Jpn. Pet. Inst. 1973, 76, 316. Yamaguchi, T.; Ohkita, S.; Kamiguchi, T. J . Jpn. Pet. Inst. 1975, 18, 278.
.
Received f o r review April 24, Revised manuscript received June 3 , Accepted June 29,
1981 1981 1981
