AN AMERICAN CHEMICAL SOCIETY JOURNAL VOLUME 8, NUMBER 2
MARCHJAPRIL 1994
G3 Copyright 1994 American Chemical Society
N
aymposwm Comparison of Liquefaction Pathways of a Bituminous and Subbituminous Coal Robert A. Keogh and Burtron H. Davis* Center for Applied Energy Research, University of Kentucky, 3572 Iron Works Pike, Lexington, Kentucky 40511 Received August 24, 1993. Revised Manuscript Received November 2 9 , 1 9 9 P
Data for the thermal liquefaction of a number of high-volatile bituminous coals suggest that these coals have a common liquefaction pathway. Verification of this pathway was confirmed using a single coal and a number of reaction conditions. The addition of a catalyst did not alter the observed thermal pathway. The thermal and catalytic liquefaction pathway obtained for a subbituminous coal was significantly different from the one obtained for the bituminous coals.
Introduction Historically, a number of approaches have been used to describe the mechanism and pathway of thermal and catalytic coal liquefaction.l-16 The liquefaction of coal is
* Abstract published in Advance ACS Abstracts, January 1, 1994.
(1)Pelipetz, M. G.;Salmon, J. R.; Bayer, J.; Clark, E. L. Znd. Eng. Chem. 19SS,47,2101. (2)Hill, G.R.; Hariri, H.; Reed, R. I.; Anderson, L. L. Coal Science;
Adv. Chem.Series No. 55; American Chemical Society: Washington, DC, 1985 D.427. ~..
(3)k&ran, G.P.; Struck, R. T.; Garin, E. Znd. Eng. Chem. Process. Des. Dev. 1967,6,166. (4)Weller,S.; Pelipetz, M. G.;Friedman, 5.Ind. Eng. Chem. 1951,43,
--.
1.57.5 -.
(5)Liebenberg, B. J.; Potgieter, H. G.L. Fuel 1973,52,130. (6)Cronauer, D. C.; Shah, Y. T.;Roberto, R. G. Znd. Eng. Process Des. Dev. 1978,17,281. (7)Shalah, M.A.; Baldwin, R. M.; Bain, R. L.; Gary, J. 0.;Golden, J. H.Znd. Eng. Chem. Process Des. Dev. 1979,18,474. (8)Abichandani, J. S.; Shah,Y. T.; Cronauer, D. C.; Roberto, R. G . Fuel 1982,61,272. (9)Guin, J. A.; Tarrer, A. R.; Pitta, W. S.; Prather, J. W. In Liquid Fuelsfrom Coal, Ellington, R. T., Ed.; Academic Press: New York, 1977; p 110. (10)Leonard, E.; Silla, H. Znd. Eng. Process Des. Dev. 1983,22,445.
a complex process and leads to numerous compounds of different types. The results of the studies on the mechanism and pathway of coal dissolution depend on how these products are defined and the number of the classes that are obtained by separations. In the early work, the primary reaction products were separated into two solubility classes, oils and asphaltenes. These were considered separately or combined to determine the reaction rates for coal. In work by Pelipetzl and Hill: a single thermal decomposition step was proposed. Further investigation~~ suggested that two distinct stages occur, with different rates of decomposition. Wellerl suggested that (11)Chen, J. M.;Schindler, H. D. Ind. Eng. Chem. 1987,26,921. (12)Yoshida, R.; Mackawa, Y.; Ishiu, T.; Takeya, G. Fuel 1976,65,
341. (13)Oblad, A. C. Catal. Rev. Sci. Eng. 1976,14(l),53. (14)Bruneon, R. J. Fuel 1979,58(3), 203. (15)Prather, J. W.; Tarrer, A. R.; Guin, J. A.; Johnson, D. R.; Neely, W. C. In Liauid Fuels from Coal: E l l i-i n , R. T., Ed.: Academic Press: New York, i977; p 246. (16)Whitahurst, D. D.; Farcasiu, M.; Mitchell, T. 0.; Dickert, J. J., Jr. EPRI FinalReport, AF-480,Electric Power Research Institute, PaloAlto, CA, July 1977.
Q887-0624/94/ 25Q8-Q289$Q4.5Q/Q 0 1994 American Chemical Society
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290 Energy & Fuels, Vol. 8, No. 2, 1994
the liquefaction of coal to produce oils was a consecutive, two-step, first-order reaction with asphaltenes as an intermediate. However, Liebenberg5 suggested that the formation of oils and asphaltenes involved two simultaneous reactions. The addition of another solubility class in more recent work, preasphaltenes (or asphatols), to the product slate of liquefaction has added complexity to the mechanisms to describe the liquefaction process. A large number of studies have been reported612 using oils, asphaltenes, and preasphaltenes, both in sequential and parallel reactions, to describe the dissolution of coal, and these have been reviewed by Oblad.13 With few exceptions,lP16the coal liquefaction process has been described using solubility classes to define the products. In this work, the solubility classes have been selectivelylumped together and plotted on a Wei-Prater" type diagram to describe the pathways of the catalytic and thermal liquefaction of a bituminous and a subbituminous coal.
Experimental Section All of the liquefaction experiments were conducted in a 50mL microautoclave reactor. The microautoclave was charged with 5 g of dried coal (-100 mesh), 7.5 g of tetralin and a steel ball (6.4 mm) for mixing. The system was pressurized with 5.5 MPa of H2(ca. 13.8 MPa or 2000 psig at reaction temperature) and immersed in a fluidized sand bath for the required reaction time. The sand bath was operated so that it required less than 2 min to reach the temperature set point. The reactor was vertically shaken at a 400 cycles/min to ensure mixing of the reactor charge. At the end of the liquefaction run, the reactor was placed in a cold fluidized sand bath. Once the reactor had been cooled to room temperature (typically within 2 min), a gas sample was obtained for analysis. The products were removed from the reactor for their analysis. The technique used for the determination of the product solubility class distributions has been described in detail elsewhere.18 The separations were accomplished using a Soxhlet extraction method. The operational definitions (on a weight percent, daf basis) used in this work are as follows: (a) oils (pentane solubles), (b) asphaltenes (benzene soluble, pentane insoluble), (c) preasphaltenes (pyridine soluble, benzene insoluble), and (d) IOM (pyridine insoluble). Conversion is defiied as 100- IOM. The reproducibility of the determination of the yields of the gases, oils, asphaltenes, preasphaltenes, and conversions are *20.02%, *1.56%, *1.06%, *0.90%, and &1.56%, respectively. Several catalysts were studied in the catalytic liquefaction experiments, A commercially available Shell 324 extrudate catalyst was ground to -100 mesh and calcined a t 723 K overnight. The ground catalyst was cooled and stored in a vacuum desiccator prior to sulfidation. The catalyst (1 g) was presulfided with dimethyl disulfide (DMDS) in the microautoclave reactor using a temperature of 658 K, a residence time of 30 min, a hydrogen atmosphere (5.5MPa, ambient), and tetralin (7.5 g) as the solvent. At the end of the sflidation run, the reactor was cooled to room temperature and vented, and 5 g of dried coal and a steel ball were added to the reactor. The reactor was purged using hydrogen and pressurized to 5.5 MPa. The liquefaction runs and product analyses were conducted as described for the thermal runs. Two additional catalysts, molybdenum naphthenate and a high surface area (270 mz/g) Fez03, were also employed. The molybdenum naphthenate was obtained from K & K Rare & Fine Chemicals. The reported Mo content was 6.07 wt % and was verified by using standard analytical techniques. The high surface area Fez03 was obtained from Mach 1. The a-FezOs catalyst has (17)Wei, J.; Prater, C. D. Adu. Catal. 1962,13,203. (18)Keogh, R.A.;Davis, B. H. J. Coal Quality 1988,7,27.
--
Asphaltcncs + Preasphaltenes (wt.%, daf)
R..ld."C.
R& ."
sva~zppm
Figure 1. Thermal liquefaction pathway of a bituminous Western Kentucky No. 6 coal. Table 1. Coal Properties WKy.No.6 W odak KCERL 71419 KCEflL 91648 proximate analysis (wt % ) ash (dry) volatile matter (daf) fixed carbon (daf) ultimate analysis (wt ?6 daf) carbon hydrogen
nitrogen total sulfur oxygen petrographic analysis (vol % ,dmmO vitrinites inertinites liptinites a
9.88 43.10 56.90
7.59 52.92 47.08
82.87 5.42 1.72 5.15 4.84
71.02 5.42 1.37 1.00 21.19
84.9 9.4 5.7
89.3 9.9 0.8
By difference.
a reported 30 A particle size. The two catalysts were sulfided in in situ using DMDS during the liquefaction experiments using the same procedures described above. A Western Kentucky No. 6 (bituminous) and a Wyodak (subbituminous) coal were used in this study. The coals were ground to -100 mesh and stored under argon prior to the liquefaction experiments. The coal properties are given in Table 1.
Results and Discussion The thermal liquefaction pathway of a Western Kentucky No. 6 coal is shown in Figure 1. In this figure, the standard solubility classes have been lumped into the following groups: oils plus gasses (0 + G), asphaltenes plus preasphaltene (A + P), and IOM (insoluble organic matter). These lumped parameters define the thermal liquefaction pathway of the bituminous coal on the ternary diagram as shown in Figure 1. The pathway of the bituminous coal has two distinct stages. In the initial dissolution stage, as the coal conversion increases, there is a parallel increase in the lumped parameter, A+P, yields. The O+G yields remain fairly constant during this stage of liquefaction. The primary reaction is therefore the conversion of coal to the intermediate lumped parameter, A+P. The second stage of the liquefaction pathway begins when a maximum has been achieved for both the coal conversion and A+P yields. In this stage, the primary reaction is the conversion of the intermediate lumped parameter, A+P, to O+G. Also during this stage of the pathway, coal conversion remains fairly constant at the maximum value. At the highest temperature studied, 718 K (445 OC)and longer residence times (15 and 30 min), there appears to be deviation from
Energy & Fuels, Vol. 8, No. 2,1994 291
Liquefaction Pathways of Bituminous Coals
0.G
It
c
Oils
I Gases (wt.%. daf)
20
40
60
I
IOM
*O
(wt.%, daf)
0
Figure 2. Catalytic liquefaction pathway of a bituminous Western Kentucky No. 6 coal.
0
660
460
860
1000
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Figure 4. Production of the lumped parameters and conversion (WesternKentucky No. 6) as a function of a severity index (SI). = '/'Rexp[
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\
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Figure 3. A first-order plot of the conversion of a Western Kentucky No. 6 coal. this stage of the pathway. The limited data available suggests that this deviation may be due to the occurrence of retrogressive reactions. In this region, the conversion, O+G and A+P yields begin to decrease. Previous work'g has shown that a large number of highvoltile bituminous coals follow this pathway. In the interest of efficiency of the liquefaction process, it would be desirable to fiid a method in which during the initial dissolution stage of the pathway, both the O+G and A+P parameters were produced as coal conversion increases and thus circumvent the observed A+P maximum yield beforea significant amount of O+G is produced. A number of different catalysts were investigated in an attempt to alter the pathway. The data obtained for the Western Kentucky No. 6 coal and two of the catalysts studied are shown in Figure 2. These data show that the pathway, and therefore the selectivity,was not changed significantly by the addition of either of the two catalysts; only the rates of the reactions were increased. In the initial dissolution stage, the conversion of the coal appears to follow first-order kinetics. The plot of the log of IOM (Figure 3) versus a severity index shows the typical straight line for a first-order reaction. The severity index (SI) is defined as follows: (19) Keogh, R. A.; Tsai, K.; Xu,L.;Davis, B. H.Energy Fuels 1991, 5, 625.
-(30000
1
1
-F ) ]
where 0 is the reaction time (minutes), OR is a reference time (5 min), T i s the reaction temperature (K), and TR is the reference temperature (598 K). The utilization of the severity index facilitates the examination of the data obtained from different reaction temperatures and times on a single graph. The behavior of the A+P, O+G, and IOM parameters as a function of the severity index is shown in Figure 4. The data in this figure show that as the IOM decreases (i.e., coal conversion increases) the A+P parameter goes through a maximum (initial dissolution stage) and then begins to decrease (second stage). The lumped parameter (O+G)remains relatively constant (in the initial dissolution stage) and then increases as the severity index increases (second stage). It should be noted that the data points in this graph and in subsequent graphs of this type are connected for clarity using standard functions in the graphics package. The observed behavior of the lumped parameters are consistent for the following series of firstorder reactions: IOM- A + P -,0 + G (2) The data suggest that, during liquefaction at the lower severity index values, a finite number of bonds are cleaved and are responsible for the oil production. In the next step of the analysis of the data, the behavior of the individual components of the lumped parameter, A+P, were examined during the initial dissolution stage. The data shown in Figure 5 indicate that both asphaltenes and preasphaltenes are produced in parallel during the dissolution stage of the pathway (SI = 0 - 140). It is apparent from these data that separating the pseudocompound, A+P, into its component parts indicates that the dissolution reaction is not a series of first order reactions which increases the complexity of the system. The data suggest that the following reactions describe the data:
The A/P ratio during the dissolution stage is shown as a function of the severity index in Figure 6. The A/P ratios of the thermal products, within experimental error
Keogh and Davis
292 Energy & Fuels, Vol. 8, No. 2, 1994 I
8oiA
45 40
t
e
I
60
0
c
360
200
IO0
400
Figure 5. Production of preasphaltene, asphaltene, and conversion (Western Kentucky No. 6) as a function of a severity index (SI).
+
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Figure 6. Asphaltene/preasphalteneratio (thermal)during the initial dissolution of a Western Kentucky No. 6 coal. have essenitally a constant value (0.64f 0.10)in this region of the pathway. During the initial dissolution stage of the catalytic liquefaction of the Western Kentucky No. 6 coal, the A/P ratio is also essentially a constant value (Figure 6 ) and is the same as for the thermal process. Thus, from these data, it follows that kz = 1.56kl and that kl + kz >> ka kq for the bituminous coal. However, from the available data, little can be said about the behavior of A and A'. If A = A , then It3 = k5; likewise, if A # A' then k3 may be unequal to kg. During the second stage of the pathway, the A/P ratios from the thermal and catalytic experiments increase with higher values of the severity index (Figure 7). Both the asphaltene and preasphaltene yields decrease with increasing severity and additional dataare needed before it is possible to distinguish between A and A' to further define the relationship between k3 and
+
0
0 0 0
S.I.
Figure 7. AsphalteneJpreasphaltene ratio during the second stage of the dissolution of a Western Kentucky No. 6 coal. Alph*llcncs t Prcasphaltencs 1wt.B. daf)
A
Figure 8. Thermal and catalytic liquefaction pathway of a subbituminous Wyodak coal. catalytic experiments are shown in Figure 8. The thermal and catalytic pathway defined for the subbituminous coal is significantly different from that defined for the bituminous coal. In the first stage of this pathway, as coal conversion increases, there is a parallel increase in both the A+P and O+G yields. The increase in the yields of A+P and O+G continues until the coal conversion approaches a maximum value. At this point in the pathway, the primaryreactions are the conversion of A+P into O+G. This stage of the subbituminous path is similar to the second stage of the pathway of the bituminous coal. Also similar is the fact that the addition of a catalyst only increasesthe rate of the reactions and does not significantly change the selectivity defined by the thermal pathway. The conversion in the first stage of the liquefaction pathway of the subbituminous coal also appears to follow first-order kinetics, as shown by the data in Figure 9. The behavior of the lumped parameters, A+P, O+G, and IOM are shown as a function of the severity index in Figure 10. As can be seen, both lumped parameters, A+P and O+G, increase initially as the IOM decreases ( f i t stage). The yields of the O+G and A+P are similar in this stage of the pathway. These data suggest the following is occurring:
k5.
A similar series of experiments were performed using a subbituminous Wyodak coal to determine the liquefaction pathway. In the initial analysis of the data, the solubility classes were also lumped into O+G, A+P, and IOM parameters. The results from the thermal and
Mo Naphthcnate
IOM
>
>
A + P (4)
O+G
where kl is approximately equal to k2. Thus it appears
Liquefaction Pathways of Bituminous Coals
Energy &Fuels, Vol. 8, No. 2, 1994 293
2.0 t
701
1bO
2b0
3bO
4b0
580
6b0
780
SbO
9bO
1bOO
S.I.
Figure 11. Production of asphaltenes and preasphaltenes (Wyodak) as a function of a severity index (SI). S.I.
Figure 9. A firat-order plot of the conversion of a Wyodak Coal. .""I
'
'
I
0.G
A
* od
li
2bO
300
460
500
do
7b0
so0
900
10'00
S.I.
Figure 10. Production of the lumped parameters, A + P and 0 + G (Wyodak)as a function of a severity index (SI).
that two parallel first-order reactions occur during this stage of the pathway. The behavior of the individual asphaltene and preasphaltene components during the first stage of the pathway are shown as a function of the severity index in Figure 11. As noted for the bituminous coal, the pathway of these individual components becomes more complex when examinedwith this method. The data in Figure 11indicate that, aa the IOM decreases, the asphaltene yields increase to a maximum and then become constant. The preasphaltene yields show a rapid initial increase and then a gradual decrease as the IOM decreases. These data suggest that during this stage the following reactions occur:
The complexity of the individual pathways of these components and the limited data make it difficult to ascertain the relationships between the individual rate constants.
Conclusions The liquefaction pathway of a high-volatilebituminous and a subbituminous coal have been defined by lumping the standard solubility classes and plotting these parameters on a ternary diagram. The thermal pathway of the bituminous coal has two distinct stages. In the initial stage there is an increase in the intermediate lumped parameter, asphaltenes plus preasphaltene, with coal conversion.Also in this stage, there is an initial production of oils plus gasses, and the amount produced remains fairly constant during this stage. The second stage of the pathway begins when the maximum in coal conversion and the asphaltene plus preasphaltene yield has been attained. The primariy set of reactions in the second stage is the conversion of the asphaltene plus preasphaltenes into oils plus gases. The lumped parameter data suggest that the conversion of the coal follows a series of first-orderreactions. However,when the pseudocompound, asphaltene plus preasphaltene, is separated into the individual components, the formation of these two intermediates do not follow a series reaction scheme. Treatment of the data in this manner shows that, in the coal dissolution stage, both asphaltenes and preasphaltenes are produced in parallel as the conversion increases. In this case, coal conversion follows a firstorder concurrent set of reactions. The thermal liquefaction pathway of the subbituminous coal is distinctly different from the one defined for the bituminous coal. In the first stage of the subbituminous pathway, both lumped parameters, asphaltenes plus preasphaltenes and oils plus gases, increase with increasing coal conversion. The second stage of the pathway begins at the maximum asphaltene plus preasphaltene yield. In this stage, coal conversion increases moderately; however, the major reaction is the conversion of the asphaltene plus preasphaltene to oils plus gases. The addition of a catalyst did not alter the liquefaction pathway defined by the thermal process for either bituminous or the subbituminous coal. The catalyst only increases the rates of reactions taking place.
