Energy & Fuels 1991,5,625-632
625
Liquefaction Pathways of U.S.Bituminous Coals Robert A. Keogh, Kanjoe Tsai, Liguang Xu, and Burtron H. Davis* Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, Kentucky 40511 -8433 Received November 19, 1990. Revised Manuscript Received May 28, 1991
The lumping of liquefaction product solubility classes (asphaltenes plus preasphaltenes, oils plus gases, and insoluble organic matter) indicates a common conversion pathway for a large number of high-volatile bituminous coals. The thermal pathway clearly shows two stages in the conversion of coal to soluble products. In the first stage, the primary reaction is the conversion of coal to the asphaltene plus preasphaltene intermediate accompanied by a relatively constant oil plus gas yield. The second stage of the pathway is initiated upon reaching maximum coal conversion. In this stage, the primary reaction is the conversion of the asphaltene plus preasphaltene intermediates to oils plus gases. A number of catalysts did not alter the pathway that was defined by the thermal conversions. Thus, catalyst addition had no major effect on selectivity, but did increase the rates of reactions. Solvent quality also had no major effect on the pathway; however, the absence of a solvent vehicle during liquefaction changed the observed pathway.
Introduction Historically, lumped parameter kinetic models1 have been used successfully to describe industrially significant complex chemical processes such as catalytic cracking: catalytic r e f ~ r m i n g ,addition ~~~ polymerization," and condensation polymerization.7~ Not surprisingly, the same approach has been used in the description of the various liquefaction processes. A physically realistic and technically viable lumped parameter kinetic model for liquefaction would be of considerable value in the development of pathways, mechanisms, and scale-up of liquefaction processes. A large number of studies have been done to elucidate both catalytic and thermal pathways of coal liquefaction. Both sequential"" and combinations of sequential and parallel reaction ~ a t h w a y s have ' ~ ~ ~been used to describe this process. With few exceptions,16the lumped parameters employed in these studies have been the solubility classes, preasphaltenes (asphotols), asphaltenes, and oils, in addition to the gases produced. In this paper, the following lumps were used to describe this process: Coal insoluble organic matter (IOM) preasphaltenes (P) asphaltenes (A) oils (0)+ gases (GI.The asphaltene and preasphaltenes (A + P)were lumped together and the oils plus gases (0 + G) were
-
-
-
Wei, J.; Prater, C. D. Ado. Catal. 1962, 13, 203. Wwkman, V.W. Lump, Modele, and Kinetice in Practice. AIChE Monograph Sir. 11, No. 11. (3) Smith, R. B. Chem. Eng. Prog. 1969,66(6), 76. (4) Brooke, B. W. Chem. Eng. 1971 March, 90-94. (6)Platzar, N. Ind. Eng. Chem. 1970,62,6-20. (6) Schulz, G. V. Chem. Tech., April, 1978, April, 220-228. (7) Flory, P. J. Principles of Polymer Chemistry, Come11University (1) (2)
-Pram: -New .- . York. 1963. (8) Cr gins, P.H. Unit PrOCeS8e8 in Organic Synthesis, 4th ed.; McGraw-%ll: New York, 1962; pp 706-709 and 718-721. (9) Weller, 5.;Pelipetz, M.G.; Friedman, 9. I d . Eng. Chem. 1961,49, ----I
1676. __ . _. (10) Rademyeki, B.; Szczygiel,J. Fuel 1984, 63, 744. (11) Suzuki,T.; Ando, T.; Watanabe,Y. Energy Fuels, 1987,1, 294. (12) Cronauer, D. C.; Shah,Y.T.; Ruberto, R. G. Ind. Eng. Chem. ROCe88 Des. Diu. 1978, 17, 281. (13) Shalabi,M.A.; Baldwin, R. M.;Bain, R. L.;Gary, J. H.;Golden, J. 0. Ind. Eng. Chem. hoce8s De8. Diu. 1979, 18,474. (14) Curran, G.P.; Struck, R. T.; Gorin, E. Ind. Eng. Chem. k'rocees. Des. Diu. 1967,6, 166. (16) Chen, J. M.;Schindbr, H.D. Ind. Chem. Res. 1987, 26, 921.
lumped together as a final product. The lumped solubility classes produced from the thermal and catalytic liquefaction of a large number of US.high-volatile bituminous coals were graphed on a triangle plot. The resulting data indicated a common pathway for all of these coals. The effects of catalyst type and solvent quality on the pathway are discussed.
Experimental Section All liquefaction experiments were conducted in a 50-mL microautoclavereactor. The microautoclave was charged with 5 g of coal (-100mesh; dried overnight under a vacuum of ca. 635 mmHg at ca. 363 K),7.5 g of the liquefactionsolvent, and a steel ball (6.4mm) for mixing. The system was pressurized with 5.5 MPa of H2(ca.13.8 MPa (ca.2000 psig) at 718 K)and immersed in a fluidized sand bath for the desired reaction time. The sand bath was operated in a manner such that it required less than 2 min to reach the desired reaction temperature. To ensure thorough mixing of the ingredients, the shaker speed (vertical) was set at 400 cycles/min. At the end of the experiment, the reactor was inserted into a cold fluidized sand bath. Once the reactor had attained ambient temperature (typicallywithin 2 min) a gas sample was obtained for analysis (H2, C1-C4, CO,). The products were removed from the reactor, using benzene for the analysis of the solubility class distribution by the Soxhlet extraction method. The technique used for the determination of the product solubility class distributions is described in detail elsewhere.l8 The operational definitions of the solubility classes used in this work (on a weight percent, daf coal basis) are as follows: (a) oils (pentane soluble), (b) asphaltenes (benzene soluble, pentane insoluble), (c) preasphaltenes (pyridinesoluble, benzene insoluble), and (d) IOM (pyridine insoluble). Conversionis defmed as (100% - IOM). The reproducibilityfor the determination of the yields of gases, oils, asphaltenes, preasphaltenes and conversions are f0.02%, f1.56%, f1.06%, 10.90%,and &1.56%, respectively. Several types of catalysts were employed in the catalytic liquefaction experiments. A commercially available Shell 324 extrudate was ground to -100 mesh and calcined at 723 K overnight. The ground catalyst was cooled and stored in a vacuum desiccator. The catalyst (1 g) was presulfided with dimethyl disulfide (DMDS)in the microautoclave, using a temperature of 658 K, a residence time of 30 min, a hydrogen atmosphere (5.5 MPa, ambient), and tetralin (7.5g) as the solvent vehicle. At completion of the sulfidation, the reactor was cooled to room (16) Keogh,R. A.; Davis, B. H.J. Coal Quality 1988, 7(1), 27.
0887-0624/91/2505-0625SO2.50,10 0 1991 American Chemical Society I
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Keogh et al.
626 Energy & Fuels, Vol. 5, No. 5, 1991
Table I. Make-up of Sample Set and Range of Key Properties B. Distribution by ASTM Rank Classification: A. Distribution of Samples by State: high-volatile bituminous A, B, C W. Kentucky 31 E. Kentucky 5 Indiana 22 Ohio 10 West Virginia 1
C. Ranges of Key Properties
minimum maximum mean
wt%
wt%
wt%
wt%
wt%
VM (daf)
C (daf)
orgS (daf)
34.27 48.34 43.15
76.04 86.48 80.67
totals (daf) 0.70 13.78 4.15
p y r s (daf) 0.03 8.87 1.70
temperature and gas vented, and 5 g of dried coal, a steel ball, and 5.5 mPa of hydrogen were added to the reactor. The liquefaction experiment and product analysis were conducted as described above for the thermal liquefaction runs. Two acid catalysts, ZnClzand SnC12-2H20,were also studied. The reactor was charged with tetralin, the acid catalyst (5 wt % of the metal chloride based on the weight of maf coal),hydrogen, and 5 g of coal on a run using the same procedure described above. An oil-soluble catalyst precursor, molybdenum naphthenate, was also utilized. The catalyst (1 g, 0.06 g of Mo) and DMDS, as the in situ sulfidation agent, was charged to the reactor along with the appropriatecoal, solvent,etc., and converted as described above. The effect of solvent quality on the observed pathway was studied by using three model compound systems as liquefaction solvents. Tetralin and l-methylnaphthalene (1-MN) were commercially available and used as supplied. The third solvent was a mixture of phenanthrene and hydrogenated phenanthrenes. The mixture was prepared by catalytically (Shell 324) hydrogenating phenanthrene;GC analysis of the product indicated the mixture was 70% hydrogenated phenanthrenes and 30% phenanthrene. The solvent mixture (7.5 g) was charged to the reactor with the appropriate coal and liquefied by using the same procedure described above. Sixty-nine coals were studied in this work. The coal samples were collected from economicseams of Kentucky, Illinois,Indiana, Ohio,and West Virginia. At each site, the samples were collected at the working face of the mine. A channel sample was collected and the seam was also benched, based on visual differences in lithotype. The coal samples were sealed in plastic bags for transportation to the laboratory for preparation. The samples were ground in air to the appropriate mesh sizes for analysesand liquefaction studies. The initial splits (6.4 mm) for liquefaction were stored under argon. Prior to the liquefaction runs,the coals were reduced to -100 mesh and stored under argon.
Results and Discussion During the past several years, CAER has had an active program to determine the liquefaction characteristics of US.coals. Approximately 300 coals were studied to determine their liquefaction potentials. As a minimum, all bench and channel coal samples were studied, using a residence time of 15 min and a reaction temperature of 658 K. The channel samples were liquefied, using two additional reaction temperatures (700 and 718 K) and the 15-min residence time. Of the 300 coals studied, samples which had a total vitrinite content greater than 80 vol 90 (dmmf) were selected for this study. The range of selected properties of these coals are given in Table I. In addition, two of the coals, a Western Kentucky No. 9 and a Western Kentucky No. 6 coal, were studied in greater detail and their complete analyses are given in Table 11. In order to examine all of the solubility class data on one graph, the asphaltene (A) and preasphaltene 8)yields and gas (G)yields were summed together to and oil (0) produce the A + P and 0 + G lumped parameters. The combining of the oil and gas yields may initially appear to be a poor choice for a lumped parameter due to the fact
0.55 4.72 2.26
vol % vitrinite 80.06 92.70 86.78
Ro0.388 0.984 0.606
Table 11. Analysis of a Western Kentucky No.6 and 9 Coal W. Ky. No. 6 W. Ky. No. 9 KCERL 71419 KCERL 71071 proximate, w t % 11.71 9.88 ash (dry) 43.21 43.10 volatile matter (daf) 56.79 56.90 fixed carbon (daf) ultimate, w t %, daf 76.80 82.87 carbon 5.41 5.42 hydrogen 1.72 1.90 nitrogen 8.41 5.15 total sulfur 7.40 4.84 oxygen' forms of sulfur, wt %, daf 2.08 2.86 organic sulfur 5.22 2.96 pyritic sulfur petrography,vol %, dmmf 84.9 90.9 vitrinites 7.1 9.4 inertinites 5.7 2.0 liptinites Ro,, a
0.77
0.54
By difference.
Figure 1. Relationship of gas yields to oil and asphaltene + preasphaltene yields (total soluble product basis) (0,658K; a, 700 K;A, 718 K). that it combines a desirable end product (oils) with an undesirable product (gases). However, in this case, where batch microautoclaves were used, the gas yields obtained for the thermal and catalytic experiments were minor contributions to the toal product yields. The percent of gases, oils, and the A + P, on a total product basis, are plotted on a triangle graph in Figure 1 for the 69 coals, using a 15-min residence time and three reaction temperatures (658, 700,and 718 K). The data clearly show that the gas yields are a minor contribution to the total products and the 0 + G lumped parameter. Thus, in the 0 + G lumped parameter, the oils are the major component so that separating the two parameters would not
Energy & Fuels, Vol. 5, No.5, 1991 627
Liquefaction Pathways of U.S. Coals
A~phalunos* Prsasphallcnos
Asphaltcncr + Prcuphnltcner (wr'k. d i n
(wi.%, daf)
A .,. .. ,, ,, ,
Figure 2. Solubility class distribution of liquefaction products using a lbmin residence time ( 0 ,658 K; A, 700 K; 0 , 7 1 8 K). change the data significantly, and therefore the conclusions derived from the data. The A + P, 0 + G, and IOM yields obtained from each product distribution were plotted on a triangle graph. The thermal liquefaction data obtained for the coals are presented in a triangle graph in Figure 2. These single residence time data suggest a common thermal liquefaction pathway. The data obtained at the 658 K reaction temperature indicate that, as the conversion of the individual coals increases, there is a parallel increase in the A + P yield, The 0 G yields remain fairly constant at ca. 10% as the coal conversion increases. These data suggest that the primary reaction is the conversion of the coal to the A P lumped parameter with a minor amount of 0 + G also produced. It is also observed that, as the coal attains a maximum conversion, the A + P yield also attains a maximum. The data obtained with the intermediate reaction temperature show a different trend. The majority of the coals have attained their maximum conversion (and A + P yield) under these conditions. In this region of the graph, the A P yields of the individuals coals decrease with a concurrent increase in the 0 + G yields, and the coal conversion remains relatively constant. The primary reaction taking place in this region is the conversion of A P to 0 + G. The data obtained by using the highest thermal severity (718K)is an extension of the data obtained by using the 700 K reaction temperature; that is a continuation of the conversion of the A + P intermediate to produce higher 0 + G yields. In addition, the liquefaction data of some coals in this region suggest the occurrence of retrogressive reactions. This results in a decrease in both the A + P yield and 0 G yield as well as the conversion of these coals. The data shown in Figure 2 suggest that the coals all have a common liquefaction pathway. The observed spread of the data at each reaction temperature is due to the large variation of coal properties represented by these coals. A number of studies have shown the effect of coal properties such as forms of sulfur, reflectance, maceral composition, etc. on coal conversions and solubility class yields.17-lg These effects are reflected in the single resi-
Figure 3. Thermal liquefaction pathway of a Westem Kentucky No. 6 coal.
+
+
+
+
7
+
(17)Yanab, R.F.;Given, P. H.; Spacmnn, W.; Davis, A. Fuel 1980, 69, 81.
(18)Snape, C.E.;Derbyshire, F. J.; Stephens, H. P.; Kattenstette, R. J.; Smith, N.W.Fuel Process. Technol. 1990,24,119. (19)Baldwin, R.M.;W e e , 5.L.; Voorhees, K. J. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1986,28(6),1.
Otis (cwt + %, 1 Gases daf)
20
40
O
60
80
hl%, dsf) M
Figure 4. Catalytic liquefaction pathway of a Westem Kentucky No. 6 coal.
dence time data shown in Figure 2. The fact is that, although maximum conversions and yields of solubility classes vary with the coals studied, they follow a common thermal liquefaction pathway. In order to verify this pathway, samples of a Western Kentucky No. 6 coal were converted, using a range of residence times and reaction temperatures. The reactor conditions were selected so that the products obtained represented the entire range of conversion possible for the coal. The data obtained in these experiments (Figure 3) verify the pathway suggested by the single residence time data in Figure 2. That is, in the initial stage of dissolution, the primary reaction is the conversion of coal to A + P. In this region of the pathway, the 0 + G yields remain relatively constant, suggesting they form very rapidly as a primary product and are not produced in high yields in the region of the pathway that convert coal directly to A + P. The pathway changes in the region of maximum conversion (and A P yield). In this region of the pathway, the major reaction is the conversion of A + P to 0 G with little, if any, increase in coal conversion. At the highest thermal severity (reaction temperature and residence time), the pathway again changes, indicating the occurrence of retrogressive reactions. This is observed by a decrease in the coal conversion and 0 + G and A + P yields. These experiments using a single coal suggest that the high-volatile bituminous coals have a common thermal liquefaction pathway. The pathway observed for the coals indicates that a maximum in the A + P intermediate is achieved prior to
+
+
(20)Derbyehire, F.J.; Davis, A.; Lin, R.; Stanaberry,P. G.; Terror, M. T.Fuel Process. Technol. 1986,12,127-141. (21)Derbyshire, F.J. Energy Fuels, l988,3,273-277.
628 Energy & Fuels, Vol. 5, No.5, 1991
Figure 5. Thermal and catalytic pathwaysof a Weatem Kentucky No. 9 coal. Asphaltones
Figure 7. The effect of solvent quality on the thermal pathway of a Western Kentucky No. 6 coal. Asphallenei * Prcasphiltencs ( ~ 1 . 4 6 ,daf)
Preasphaltenes
(wt.%, daf)
011s (wt %,
dan
( w t %.
daf)
Figure 6. Thermal and catalytic pathways in the presence of a nondonor (1-MN) solvent ( 0 ,668 K; A, 700 K; 0, 718 K).
+
a substantial increase in the 0 G yields. It is desirable to alter the pathway in such a manner that the 0 G yield increases as coal conversion increases. A supported catalyst (Shell 324) and an oil-soluble catalyst precursor (molybdenum naphthenate) were utilized to determine if these added catalysts effect the pathway to produce higher 0 G yields as the coal conversion reaches a maximum, by circumventing the observed A + P maximum. The same Western Kentucky No. 6 coal was used in these catalytic liquefaction experiments. The results of experiments with the Shell 324 and molybdenum naphthenate catalysts are shown in Figure 4. The pathway defined by using both the supported and unsupported catalysts is similar to the observed thermal pathway for this coal. Therefore, the major effect of catalyst addition on the pathway is to increase the rate of production of the intermediates, A P and has no major effect on the selectivity defined by solubility classes as discussed above. The effect of two acid catalysts, ZnClz and SnC12*2H20, on the pathway was studied by using a Western Kentucky No. 9 coal. The data (Figure 5) clearly show that the thermal and catalytic pathways for this coal are also very similar. The acid catalysts increase the rate of production of the A + P intermediate and do not significantly change the selectivity. The effectiveness of a catalyst may be influenced by the presence and quality of the liquefaction solvent.= The
+
+
+
(22) Weller, S.;Peliptz, M.C.; Freidman, S.;Storch, H. H. Znd. Eng. Chem. 19SO,42,330-334. (23) Hawk, C. 0.;Hiteahue, R.W.US.B w . Mine0 k l l . 1966, No.633, 195.
Figure 8. Thermal pathway of a Westem Kentucky No. 6 coal wing a mixture of phenanthrene and hydrogenated phenanthrenes.
thermal liquefaction data obtained with a number of Eastern and Western Kentucky coals, a 15-min residence time, 1-MN (nondonor) solvent, and three reaction temperatures (658, 700, 718 K) are shown in Figure 6. Although the majority of the data was obtained by using the 658 K reaction temperature, the thermal liquefaction pathway is common to the one shown in Figure 2 using tetralin (donor) as the solvent. The only difference observed between the single residence time data obtained by using tetralin and 1-MN is that the maximum conversions (and A + P yields) are significantlylower in the nondonor solvent. Therefore, the donor ability of the liquefaction solvent does not effect the common thermal pathway for these high-volatile bituminous coals at the lower severity conditions. The liquefaction pathway defined by the product slate obtained by using the combination of the acid catalyst SnC12-2H20and 1-MN is compared to that obtained thermally by using tetralin and 1-MN (Figure 7) for a Westem Kentucky No. 9 coal. A common pathway applies for the three systems and is common to the pathway previously discussed. The thermal pathway for a phenanthrene (30wt 9%)and hydrogenated phenanthrenes (70 wt %) mixed solvent and the Western Kentucky No. 6 coal is shown in Figure 8. A similar pathway is defined for the coal utilizing this solvent system as that defined by tetralin and 1-MN. Although the pathways are similar for the different liquefaction solvents, the time to attain the different maximum conversions (and A + P yields) varies with the solvent. The catalytic pathway in the absence of solvent is shown in Figure 9. The Western Kentucky No. 6 coal was liquefied using the SnC12-2H20catalyst in a hydrogen at-
Energy & Fuels, Vol. 5, No.5, 1991 629
Liquefaction Pathways of U S . Coals
.
Arphaltcncr Proaaphaltcncs (wi.%, daf)
.
,*
I
85..
807s.70.65.. 60.. 5550-
--
0
900 0
0
.
0
I
'
45-
40.. 35.. 30.1 Oils
.
Gases
20
60
40
80
25-9 20.?
IOM (w%,daf)
(.ut.%, dnf)
153
Figure 9. Catalytic pathway of a Western Kentucky No. 6 coal
in the absence of a solvent.
mosphere. The absence of a solvent changes the liquefaction pathway. In the initial stage of conversion, the 0 G yields increase with increasing conversion. The A + P yields remain fairly constant during this stage of the pathway. This is the opposite trend observed with the liquefaction solvents previously discussed. Similar to the previously discussed pathways, when conversion reaches a maximum, the pathway changes and the conversion begins to decrease. Parallel to the conversion decrease, there is a decrease in the A + P yields and the 0 + G yields remain constant. In this region of the pathway, the data suggest that the A + P intermediate is undergoing retrogressive reactions while the 0 + G yields remain constant. The 0 + G yields are similar to that obtained by using the solvent vehicles discussed above. In their early dry hydrogenation work, Weller22and Hawk23also report high distillate yields. In his pioneering work, Bergius also obtained a high liquid yield when a solvent was not used; however, he was forced to use a solvent in larger scale work in order to prevent excessive temperature increases. It seems likely that a t least a part of the increased oil yield in the smaller reactor runs may be due to localized exotherms. One of the simplest kinetic models (eq 1)represents coal liquefaction as a series reaction scheme, involving the sequential formation of preasphdtenes, asphaltenes, and oils from the reactive maceral fraction of the parent coal:24
+
coal
k1
preasphaltenes
kl
asphaltenes
kS
oils
(1)
Variations of this model abound and are too numerous to be reviewed completely here. Pelipetz et a1.% and Hill et a1.% assumed one thermal decomposition. Weller et alan proposed that the conversion of dto oils irivolves two consecutive first-order reactions with asphaltenes as an intermediate product. A third class of compounds, the preasphaltenes, have been isolated from liquefaction products and have been identified as a key reaction intermediate.% Thus, Schweighardt et alaaproposed a more complex model using (24) Shalabi, M.A.; Baldwin, R. M.;Bain, R. M.;Gray, J. H.; Golden, J. 0. Ind. Eng. Chem. Process Des. Dev. 1979,IS,474-479. (25)Pelipetz, M. G.; Salmon, J. R.; Boyer, J.; Clark, E. L. Ind. Eng. Chem. 1966.47.2101-2103. , ~_ _ ~. _ . ~ (26)Ha1,'G. R.; Hariri, H.; Redd, R. I.; Anderson, L. L. Coal Science Adu. Chem. Ser. 1966,55,427-447. (27)Weller, 5.;Pelipetz, M.G.; Friedman, S . Ind. Eng. Chem. 1951, 43., 1675-1679. ~ .. _. .. (28)Neavel, R. C. Fuel 1976,55,237-241. (29) Schweighardt, F. K.; Retcofsler, H. L.; Raymond, R. P r e p . Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1976,21,27-32. ~~
P
.,
Figure 10. Thermal conversion of a Western Kentucky No. 6 coal a~ a function of a severity index (0,598 K 0,658 K, 0,700 K; 718 K).
preasphaltenes in addition to oils and asphaltenes. Subsequently, simplified versions of this model have been satisfactorily tested with data from batch reactors as well as continuous reactor units. Mohan and Silla,30for example, employed coal liquids separated into solubility classes to test their data with several kinetic models and summarize the literature on coal liquefaction mechanisms based on solubility classes. Pina-Aviles et al.31932 showed that temperature has a positive effect (i.e., increase in conversion with increase in temperature) for the formation of pentane and toluene solubles. Mohan and Silla30 also observed this effect in nonisothermal experiments. On the other hand, Abichandani et alea reported a negative effect of temperature on the formation of pentane solubles. They attributed this effect to an enhancement of the adductive reactions between coal and oils yielding pentaneinsoluble products and found this negative effect to be most pronounced at 2 min of reaction time and to decrease with increase in time. Pina-Aviles et al.31132treated liquefaction as a simple series reaction as was done by Shalabi et aL2'; however, two coal-dependent parameters were utilized. Thus, the initial coal concentration was adjusted by subtracting the amount of extractable mobile phase obtained by using pyridine and a Soxhlet apparatus. In addition, the amount of coal that remained unconverted at long reaction time was subtracted from the amount of coal added to the reactor. These two corrections improved the fit of the data to the integrated rate expressions. Gollakota et al.s4recently utilized the simple series reaction mechanism; however; these authors included a term that permitted an amount (a)of the process solvent to combine with coal to produce preasphaltenes. These authors concluded that both thermal and catalytic reactions proceeded in parallel and that the progressive series reaction sequence satisfactorily represented both the themal and catalytic liquefaction of coal. Satowrecently corrected the equations utilized by Gollakota et al." since the latter (30)M o b , G.; Silla, H. Ind. Eng. Chem. Process Des. Dev. 1981,20, 349-358. (31)Pina-Aviles, B.; Collins, D.; Snell, G.; Davis, B. Min. Metall. Process. 1985,2, 154-159. (32)Pina-Aviles, B. M.S.Thesis, University of Louisville, 1984. (33)Abichandani, J. S.;Shah, Y. T.; Cronauer, D. C.; Ruberto, R. 0. Fuel 1982,61,276-282. (34)Gollakota, S. V.;Guin, J. A.; Curtis, C. W. Ind. Eng. Chem. Process Des. Deu. 1986,24,1148. (35)Sato, S . J. Chem. Eng. Jpn. ISSO,29,249.
Keogh et al.
630 Energy & Fuels, Vol. 5,No.5, 1991
:'I
t
55 * O
30
.
I
I
b
'I 10
IO 8
.O 0
5
6
0 0 0
400
200
600
800
1000
1200
1400
1600
1800
200
400
600
Severity Index
Figure 11. Thermal preasphaltene yields as a function of a 598 K; 0 658 K 0,700 K B, 718 K). severity index (0, 44 42 40 38 36 34 32 2 30 28 " 26 24 9 22 2 20 9 18 16 14
100 95
90
0
'
.
0
.* .
'
85 80
a m b
1200
1000
1400
1600
1800
Figure 13. Thermal oil plus g a m yields as a function of a seventy index (0,598K; O , 658 K; 0,700 K; 718,K).
I
.
800
Severity Index
.-
75
f
65
70
i
598 K
658 K
i :
,
" :
700 K w o
o
718 K
s joo
Q
t
o
I
:
o
+
4 0 i
"I
0
0
ooo 0
00
:
20
1s
6 4 6 0
10 5
200
400
600
800
1000
1200
1400
1600
1800
0
Scrvcrity Index
Figure 12. Thermal asphaltene yields as a function of a severity index (0,598 K; 0,658 K; 0,700K;m, 718 K).
authors did not provide mass conservation in their calculations. Sato%considered that the model without solvent incorporation provided a better fit to the coal liquefaction data than the correct kinetic expression for solvent incorporation. In situations such as with coal liquefaction, an appreciable conversion may occur during the heat-up and/or cool-down time. Thus, conversion will be a function of both time and temperature during the nonisothermal period($. This has been described by various functions of time and temperature.% For example, a severity index (SI) may be defined as follows:
where 8 is the reaction time (minute), 8, is a reference time (5 min in this case), T is the reaction temperature, and TR is the reference temperature. As is evident from Figures 10 and 11, the total conversion and weight percent preasphaltene production show the expected trends for conversion of a reactant, coal, and the production of the first intermediate, preasphaltene which quickly attains a maximum yield and then declines with increasing SI. However, the curves for the production of the next in(36)Varghese, P.; Derbyshire, F.; Whitehumt, D. Presented at the Symposium on Instrumentation arid Control for Foeail Energy Proceeses June 9-11,1980.
................................
io
40 60 80 20 Residence Time (min.)
40
60
20
40
60
Figure 14. Thermal conversion of a Western Kentucky No. 9 coal using a number of reaction conditions.
termediate, asphaltenss (Figure 12), and the final product, oils plus gases (Figure 13), show considerable scatter and the aaphaltene curve does not attain a maximum yield as expected for a consecutive reaction scheme: coal
-
preasphaltene
-
asphaltene
-
oil + gas
(3)
Thus, the severity index does not provide a very satisfactory framework to account for all of the experimental data. Another approach is to consider that the bonds undergoing cleavage during the conversion are composed of a subset of bond types, each subset with an "average activation energy". Thus, at each reaction temperature, a significant fraction of the conversion will occur during the short time interval (2-4 min) of the approach to the isothermal state; this conversion will result from rupture of those bonds with low activation energy. Thus, at 598 K, about 15 % conversion is attained during heat-up and an additional ca.15% conversion occurs during the next hour (Figure 14). A similar situation occurs a t 658 K except now about 30% conversion occurs during the heat-up period and then the maximum conversion of ca. 80% is attained during the next 90 min. The data for the reaction at 700 K follows a similar trend; however, for the highest reaction temperature (718 K) the maximum conversion is essentially attained during the heat-up period. If we apply the SI equations for the 658 and 700 K reaction temper48700.Thus, we should anticipate atures we find that 19-
-
Energy & Fuels, Vol. 5, No. 5, 1991 631
Liquefaction Pathways of US.Coala 40 38
..
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ken. Thus, the fraction of coal converted to asphaltenes and preasphaltenes during heat-up will depend upon the isothermal reaction temperature. Hence,
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Figure 17. Thermal oils plus gases yields using a number of
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Figure 16. Thermal asphaltmeyields using a number of reaction
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the same conversion at 5 min at 700 K and 20 min at 658 K; this is not the situation that is observed in Figure 14. The preasphaltene yields (Figure 15) are likewise consistent with the conversion during the heat-up period being greater than anticipated from the conversion during the rest of the reaction. The yields of asphaltenes (Figure 16), except for the 718 K case where retrogressive reactions may occur,appear to be consistent with this picture. Likewise, the yields of 0 + G (Figure 17) are consistent with this viewpoint. It therefore appears that it is more preferable to view the conversions as consisting of a series of reactions, each subset of reactions with its own activation energy, rather than a conversion that can be treated by a single kinetic equation. Coals of similar rank show a range of conversions for a constant residence time, e.g., 15 min. If the subset of reactions applies in this case, the more reactive coals would consist of larger subsets of bonds with lower activation energies than for those coals that were less reactive. Thus, our data are consistent with the first step of coal liquefaction producing 5-10 w t % of oil in an initial burst that is essentially independent of both reaction time and temperature. The remaining fraction of reactive coal is converted to asphaltenes and preasphaltenes; during this conversion some fraction is converted during the heat-up period. The fraction of coal that is converted to asphaltenes and preasphaltenes is viewed to be composed of structures that are defined by bonds whose strength requires increasingly larger activation energies to be bro-
At the highest reaction temperature, 718 K, pi = 1 - a I, where I k the fraction of inerta in the coal. The situation is viewed to be different at lower temperatures. Thus pw is viewed to consist of two terms (5) p598 = OH-U -k Bt where represents those bonds that are sufficiently weak for essentially complete conversion to occur during heat-up and & represent a subset of stronger bonds which undergo conversion during the isothermal time period at a measurable rate. For the next higher temperature 658 K, the subset of bonds that undergoes rupture during heat-up is essentially equal to all bonds ruptured during 60 min at 598 K, Page. During the 90-min reaction time at 598 K, another subset of stronger bonds undergoes conversion during the isothermal time period a t a measurable rate. Thus, the number of subsets defined by bond strengths that convert during heat-up, &-u, and the subset with a sufficiently high activation energy to represent the kinetic conversion, &, depend upon the temperature of the isothermal conversions. The conversion profiles involved here require the use of hydrogen from the solvent and/or gas phase. It is therefore inviting to speculate that two reactions are primarily responsible for the conversion profiles: (1)the reactions involving reduction in molecular weight are thermal reactions even in the presence of a catalyst and (2) hydrogenation reactions which occur slowly during the thermal reaction occur more rapidly in the presence of a catalyst. If this viewpoint is correct, hydrogenation is a slower reaction than the thermal pyrolysis reactions that effect molecular weight reductions. Since the reaction pathway considered here is based upon solubility classes, i.e., molecular weight distributions, we should observe a common thermal and catalytic pathway if a thermal reaction is responsible for molecular weight reduction in both cases. Conclusions Combining the liquefaction product solubility classes into the groups A + P, 0 + G, and IOM and plotting the
632
Energy & Fuels 1991,5,632-637
data on a triangle graph clearly show a common pathway for the high-volatile bituminous coals used in this study. In the initial stage, as the conversion increases there is a parallel increase in the A + P yields. The 0 + G yields remain fairly constant in this initial dissolution stage. Upon reaching a maximum coal conversion, the A + P yield also reaches a maximum and the pathway changes direction. In this second stage of the pathway, conversion remains relatively constant and the 0 + G yields increase at the expense of the A + P yields. At severe process conditions (high temperatures and long residence times), the pathway again changes. In this region of the pathway, total conversion decreases and lower yields of 0 + G and A P are obtained. Retrogressive reactions are the primary reactions taking place in this section of the pathway. The utilization of supported, oil based, and acid catalysts has no major effect on the pathway observed for these coals. The catalyst therefore only increases the rate of reaction and does not significantly change the selectivity. The quality of the liquefaction solvent has no major effect on the thermal or catalytic pathway. The primary difference in the observed pathway for the different liquefaction solvents is the magnitude of the maximum
+
conversion obtained for the coals. The better the donor ability, the higher the maximum conversion obtained by the coal. The absence of a liquefaction solvent in the catalytic hydrogenation of a Westem Kentucky No. 6 coal changed the pathway. In this experiment, the 0 + G yields increased with increasing coal conversion in the initial section of the pathway; the A + P yields marginally increased. This is opposite to the trend in liquefaction with a solvent. Similar to the pathway observed in the presence of a liquefaction solvent, when the coal conversion reaches a maximum, the pathway changes. Upon reaching the maximum coal conversion, the pathway indicates retrogressive reactions are occurring. The coal conversion and A + P yields decrease in this region of the pathway; however, the 0 + G is comparable to those obtained in the presence of a solvent vehicle.
Acknowledgment. This work was supported by the Commonwealth of Kentucky and DOE Contract No. DEFC22-88PC8806 as part of the Consortium for Fossil Fuel Liquefaction Science (administered by the University of Kentucky).
Determination of Atomic Groups of Hydrocarbons in Coal-Derived Liquids by High Performance Liquid Chromatography and Nuclear Magnetic Resonance? Masaaki Satou,* Hirofumi Nemoto, Susumu Yokoyama, and Yuzo Sanada Metals Research Institute, Faculty of Engineering, Hokkaido University, N-13 W-8, Kita-ku, Sapporo, 060 Japan
Received January 23, 1991. Revised Manuscript Received June 17, 1991
A systematic atomic group determination by high performance liquid chromatography (HPLC) and nuclear magnetic resonance (NMR) is proposed for the estimation of physical properties of hydrocarbons in a coal-derived liquid. HPLC has been used for the separation of hydrocarbons into compound classes according to the number of aromatic rings. The structural analyses of these compound classes were performed by using 'H and I3C NMR. The chemical structures of the compound classes are characterized by seven atomic groups. The numbers of these atomic groups in an average molecule for each fraction were calculated based on the resulta from this method. The physical properties of coal-derived liquids will be estimated from the chemical structure of the hydrocarbons by the method proposed here. Introduction Extensive data about the physical properties of coalderived liquid are necessary for the optimum design and operation of coal liquefaction and upgrading processes. However, the data are generally available only for low molecular weight hydrocarbons. They are more scarce especially for hydroaromatic and polyaromatic hydrocarbons, which exist in significant amounts in coal-derived liquids. Determination of all required data is usually not convenient experimentally. Even if possible, the wide variety of physical properties that are commonly measured -
~~
~
'Presented at the Symposium on Analyticd-Chemistry of Heavy Oile/Reside, 197th National Meeting of the American Chemical Society, Dallas, TX, April 9-14, 1989.
generally dictates a wide range of testing procedures, many of which can be quite tedious. Hence, a correlation is needed to estimate the values or to extend or extrapolate the limited available data. In general, the physical properties are closely related to the chemical structures of a given heavy hydrocarbon molecule.' One of the most common approaches to physical property prediction has been a group contribution method in which one assumes that the physical properties of a molecule are determined by the number and types of chemical groups p r e ~ e n t . ~ sThis method for the coal(1) Benron, 5.W. Thermochemical Kinetice, 2nd ed.; Wiley: New
York, 1976.
(2) Reid, R. C.; Prauanitz, J. M.; Sherwood, T. K. The Propertiee of Gases and Liquids, 3rd ed.; McGraw-Hill: New York, 1977.
0 1991 American Chemical Society
