202
Ind. Eng. Chem. Fundam. 1984, 2 3 , 202-207
Effect of Nitrogen Content of Solvent on Coal Liquefaction Sudhlr V. Panvelker, Welrong Ge, and Yatlsh T. Shah Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 1526 1
Donald C. Cronauer' Gulf Research & Development Company, Pittsburgh, Pennsylvania 75230
SRC-I1 recycle solvents contain substantial amounts of heterocyclic compounds such a s quinoline; hydrogenated derivatives of such compounds can be good Hdonors. In addition, hydrogenation of quinoline to tetrahydroquinoline is faster than that of the corresponding homocyclic aromatic compound such as naphthalene. Hence, such compounds should aid in maintaining the H-donor capability of recycle solvents at high levels during all phases of coal conversion. In a series of coal liquefaction runs made to test this premise, there was no effect of tetrahydroquinoline addition to hydrogenated phenanthrene and SRC-II solvent on the product distribution from coal liquefaction. However, there was a very pronounced effect with anthracene oil as solvent. Adduction of tetrahydroquinoline led to a net loss of solvent and a very high yield of asphaltenes. These results indicate the formation of adducts through the reactions of solvent and coal species. The level of adduction lessened with extended reaction times, indicating some reversibility.
Introduction In recent years, a large amout of research has been carried out on the fundamental understanding of the mechanism of direct coal liquefaction wherein coal is converted into clean burning solid and liquid fuels. Solvent refined coal processes (SRC-I and SRC-11), the Exxon donor solvent process (EDS), and the H-Coal process are the typical processes developed for this purpose. Reviews describing the chemical and the engineering aspects of these processes are available: see, for example, Whitehurst et al. (1980) and Shah (1981). Although the nature and the quality of products, the operating conditions, and the severity of liquefaction vary from process to process, the underlying principles for the mechanism of coal liquefaction are the same. This paper provides some insight on the effect of nitrogen content on the mechanism of coal liquefaction. Coal can be considered as a highly cross-linked amorphous macromolecular network consisting of relatively stable aggregates linked together by weaker linkages. These linkages are usually either aliphatic (C,-C3) or involve heteroatoms such as oxygen or sulfur. When coal is contacted with a solvent in the presence of H2 a t high temperature and pressure (700-745 K, 6.9-35 MPa), the solvent penetrates the pore structure of the coal particles, causing it to swell and fracturing the labile cross-links in the coal molecule. This fracturing involves both the breaking of hydrogen bonds and, as the temperature increases, the thermal cleavage of chemical bonds. The cleavage of these bonds yields reactive free radicals as noted by Curran et al. (19671, Neavel (19761, and Whitehurst et al. (1977). If there is hydrogen available to cap off these free radicals, stable species with molecular weight typically ranging from 300 to 1000 are obtained. The hydrogen may be available from the solvent components (Curra et al, 1967),the hydrogen-rich portions of coal itself (Collins et ai., 19771, or the dissolved hydrogen (Vernon, 1980). If no hydrogen is available, the free radicals can recombine to form high molecular weight products ultimately leading to char. A good solvent should inhibit the formation of heavy products. For this function, it must contain components which have labile hydrogen to quench the free radicals.
These components, once hydrogen is depleted, should accept hydrogen from the gas, thereby maintaining the hydrogen-donor capacity of the solvent at high levels at all phases of coal conversion. Usually, partially hydrogenated aromatic hydrocarbons such as Tetralin are considered to be "good" constituents of the solvent. However, as shown by Whitehurst et al. (1980), rehydrogenation of depleted aromatic solvent components such as naphthalene back to Tetralin does not occur to an appreciable extent in SRC processes. This is not because of any thermodynamic limitations, but because of small rates of hydrogenation even in the presence of catalytically active mineral matter components. Schiller (1977) showed that recycled coal liquefaction solvents also contain heterocyclic hydroaromatics such as tetrahydroquinoline. Brucker and Kolling (1965) and Hausigk et al. (1969) demonstrated that these compounds are effective as hydrogen donors. Hydrogenation of a heterocyclic aromatic such as quinoline is relatively easier than that of a homocyclic aromatic such as naphthalene. Quinoline equilibrates very rapidly to 1,2,3,4-tetrahydroquinoline in the presence of commercial hydroprocessing catalysts a t around 600 K (Shih et al., 1974). However, Sapre and Gates (1969) pointed out that naphthalene hydrogenates very slowly under similar conditions. This relative ease of conversion of quinoline is also observed in the presence of coal mineral matter (Panvelker, 1982). In a previous study with model compounds representing coal linkages and functional groups, Panvelker et al. (1982) demonstrated that tetrahydroquinoline was a better Hdonor than Tetralin. The present study was undertaken to evaluate the effect of tetrahydroquinoline addition to a solvent during the liquefaction of Pittsburgh Seam bituminous coal at typical reaction conditions. This coal was chosen as part of an on-going DOE-supported study, and the results should be comparable to those of other bituminous coals. The effects of tetrahydroquinoline upon the liquefaction of subbituminous coals having higher oxygen contents maybe somewhat different. Experimental Section The experiments were carried out in a 1-L stainless steel autoclave supplied by Autoclave Engineers, Inc. The au-
0196-4313/84/1023-0202$01.50/00 1984 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 23, No. 2, 1984 203
Table 11. Analysis of Solvents'
0'" 0'" 0'"
V.nt
element carbon hydrogen oxygen nitrogen sulfur
anthracene oil SRC-I1 solvent hydrophenan- prior t o after prior t o after threne, treattreattreattreat% ment, % ment, % ment, % ment, % 91.08 8.8 0.01 small
91.17 5.81 1.33 1.03 0.63
91.72 6.32 1.38 0.27 small
87.25 8.69 2.82 0.88 0.36
88.94 8.75 1.90 0.23 small
Note: Treatment consisted of mixing the solvents in aratio of about 4 : l with Amberlist A-15 ion-exchange resin overnight t o partially remove basic nitrogen compounds.
v
Volv.
F T F w d tank
Po Rorsurr gougo TI Tomp.r8turo lndicitor
Figure 1. Schematic of the batch experimental unit. Table I. Analyses of Powhatan No. 5 Coal A. proximate analysis (dry basis) ash volatile matter fixed carbon B. ultimate analysis (dry basis) carbon hydrogen nitrogen chlorine sulfur oxygen ash C. vitrinite content D. initial tetrahydrofuran (THF) solubility a
9.7% 40.1% 50.2% 72.3% 5.1% 1.5% 0.03% 3.6% 7.9% 9.7% 89.2 vol % 10.25%=
Moisture-ash-free basis.
toclave was equipped with a magnetic stirrer. Figure 1 shows a schematic drawing of the experimental setup. The solvent was initially charged to the autoclave and heated to a temperature 15 K above the desired temperature after pressurizing with H2to an initial pressure of 2.0 MPa. The ground coal which was charged to a separate feed vessel was then injected rapidly into the autoclave by opening the valve V4 in Figure 1. This instant was marked the zero time for the reaction. The temperature dropped by about 50 K on injection but recovered with 1min and was then maintained within f l K about the desired temperature. The reactor pressure was maintained at 10.34 m a . Liquid samples were withdrawn at predetermined time intervals up to 60 min. For all experiments,the stirrer was operated at 1500 rpm-a speed sufficiently high to ensure mixing in the reactor and eliminate possible mass transfer resistances. Powhatan No. 5 from Pittsburgh Seam No. 8, a high volatile bituminous coal, was used in this study. Table I shows the analyses of this coal. The coal was ground to pass 97% through a 100-mesh sieve. Three different liquefaction solvents were used. A mixture of hydrogenated phenanthrene compounds (HPh) as obtained by partially hydrogenating phenanthrene (supplied by Aldrich Chemical Co.), at 683 K, 10.34 MPa in the presence of CoMo catalyst. The technique was similar to that of Ruberto (1980). Anthracene oil, a coke-oven product, was supplied
by Reily Tar and Chemical Corporation. SRC-I1 solvent was a process-derived solvent obtained from a pilot-plant run at Gulf Research Center, Harmarville, PA. Hydrophenanthrenes had a negligible nitrogen content. Anthracene oil and SRC-I1 solvent were partially denitrogenated by treating with Rohm & Haas Amberlist ionexchange resin. Table I1 gives elemental analysis of these solvents. The product samples were sequentially extracted by use of a Soxhlet apparatus with pentane, toluene, and tetrahydrofuran (THF) to evaluate the products in terms of oils, asphaltenes, and preasphaltenes, which are defined as follows. wt % oils = 100% [amount insoluble in pentane/MAF coal] X 100 wt % asphaltenes = [(amount soluble in toluene but insoluble in pentane)/MAF coal] X 1CO
w t 70 preasphaltenes = [(amount soluble in THF but insoluble in toluene)/MAF coal] X 100 Due to the type of reactor, no attempt is made to measure the yield of gases, and from the above definition, "oils" are classified as all of the coal product that is not insoluble in pentane as determined by Soxhlet extraction. While this includes liquid products, it also includes gases and other byproducts such as ammonia, hydrogen sulfide, water, etc. In the event that the extent of adduction to the three heavier fractions gives a pentane-insoluble fraction that is greater than the MAF feed coal, a negative value of oil yield is obtained. It is also noted that a major portion of the volatile components were stripped prior to the extraction, so cosolvent effects were small in these extractions. Reproducibility of these results (5% oils, etc.) were typically within f 3 w t 90 (based on these and similar runs). These lines were drawn only to show trends. Results and Discussion Hydrophenanthrene, SRC-I1solvent, and anthracene oil had very different H-donor capabilities. Hydrophenanthrene contained partially hydrogenated three-ring compounds such as dihydro-, tetrahydrophenanthrene, etc., all of which had very labile hydrogen. Hence, hydrophenanthrene was a "good" solvent. On the other hand, anthracene oil was predominantly aromatic and did not contain an appreciable quantity of hydroaromatic hydrogen. Polyaromatic compounds are capable of converting coal into soluble products without any hydroaromatics being present (Davis et al., 1977), because such compounds aid in redistributing hydrogen among different coal species. This phenomenon is important in the early stages of coal dissolution. However, the quantity of labile hydrogen available from coal is limited, and hence the beneficial effect of such polyaromatics is short lived. Seshadri et al. (1978) have presented a method for estimating the
204
Ind. Eng. Chem. Fundam., Vol. 23, No. 2, 1984 1 .o
Table 111. Run Conditions
solvent
coal/ solvent (moisturefree)
hydrophenanthrenes (HPh) HPh + 9.98% THQ HPh + 19.45% THQ treated SRC-I1 treated SRC-I1 untreated SRC-I1 treated SRC-I1 + 10.37% THQ treated SRC-I1 + 19.98% THQ treated SRC-I1 + 20.19% THQ treated anthracene oil (AO) A0 A 0 + 10.00% THQ A 0 + 19.25% THQ A 0 19.97% THQ
0.52 0.48 0.50 0.46 0.45 0.51 0.56 0.69 0.69 0.51 0.40 0.48 0.35 0.22
run no. H1 H2 H3
SI s2 s3
S4 s5 S6
A1 A2 A3 A4 A5
-
2
0.8
0 b
iL
4
" . 0
.
0.6
u J II _1 3
?.
I
Y
5
-
0.4
7
E i
0 0 i
k
0.2
r
SYMBOL
RUN
0
0.40
:
HI
10
H3
30
20
60
70
YINLTES
Figure 3. Effect of the addition of THQ to hydrophenanthrenes on the conversion to toluene solubles. l.O
~
0.20
SYMBOL
RUN
SOLVENT
- - NO. 0.8
0.10
0.52 0.48 0.52
50
40
REACTION ? l P E ,
0.30
MF COAL SOLVENT
HPH HPH+lO%THQ HPH+l9.5%THQ
HZ
0.0
0
SOLVENT
- - NO.
1
0.00 0
0 D
Hl
SYMBOL
RUh
SOLVENT
MF COAL SOLVENT
I
I
I
1
I
I
10
20
30
40
50
60
REACTlOh TIME,
0.52 0.48 0.52
HPH HPH+lO%THQ HPH+19.5%lHQ
H2
H3
NO. _ _ -
MF C O A L SOLVENT
I
I
1
70
MINUTES
Figure 2. Effect of the addition of THQ to hydrophenanthrenes on the conversion to oils.
transferable hydrogen from such solvents. The relative indices indicating transferable hydrogen from anthracene oil, SRC-I1 solvent, and hydrophenanthrene, as calculated by this method, were 0.15, 0.80, and 2.25, respectively. When anthracene oil and SRC-I1 solvent were treated with resin, about 75% of the nitrogen was removed, p!esumably basic. Different amounts of tetrahydroquinoline (THQ) were then added to these solvents to change their nitrogen content. Table I11 shows the nature of the solvent and the coal/solvent ratio used for each run carried out in this study. Coal was liquefied a t 711 K with anthracene oil and a t 723 K with SRC-I1 solvent and hydrophenanthrene. In all experiments, the reactor pressure of 10.34 MPa (H,) was used. It was not possible to maintain an identical coal/solvent ratio, as some coal was left behind in the feed tube after injection. Figures 2 through 4 show the yields of oils, toluene solubles (oils plus asphaltenes), and coal conversion in terms of THF insolubles, as a function of time, for the case of the hydrophenanthrene solvent, with and without added THQ. For this case there was no effect of THQ on the product distribution except, perhaps, at short times for the production of oils. This result is not totally unexpected, because hydrophenanthrene has a very high H-donor capability, and an addition of THQ would not alter this
0
0.0
I
0
I
I
I
I
I
I
I
10
20
30
40
50
60
70
REACTION TIME,
MINUTES
Figure 4. Effect of the addition of THQ to hydrophenanthrenes on the conversion to THF solubles.
ability to a great extent. Furlong et al. (1976) showed that for very high values of available donor hydrogen, the liquefaction performace is independent of H-donor availability. Hydrogen from more potent donor components of hydrophenanthrene, such as dihydrophenanthrene, may be as labile as that from THQ. Figures 5 through 7 show similar plots for the SRC-I1 solvent. There is some scatter in the data. The only trend that is observed is that the yields of both pentane and toluene solubles are marginally lower for the raw (untreated) SRC-I1 solvent than for the other solvents. However, this is within the scatter of the data. Thus, the addition of THQ was ineffective in altering the yields of extractables when using SRC-I1 solvent. This solvent contained much less available hydrogen than the hydrophenanthrene. It is difficult to say if addition of THQ would enhance its H-donor properties. The increase of
-
20
0.40
c
-SYMBOL
LL
a
,z
RUN NO.
@
51
X
+
52 53
+
54 55 56
SOLVENT
-
M F COAL SOLVENT
TREATED S R C - I 1 TREATED S R C - I I UNTREATED S R C - I I T ' D SRC+IO.4%THQ T ' D SRC+ZO.O%THQ T ' D SRC+ZO.Z%THQ
0.46 0.45 0.51 0.56 0.69 0.69
SYMEOL
+
0.8
-a
Ic
51
X
52 53 54
+
2
i2 t
RUN
0
SOLVENT
M F COAL SOLVENT
T R E A T E D SRC-I1 TREATED S R C - I I UNTREATED S R C - I I T ' D SRC+10.4%THQ T ' D SRC+ZO.D%THQ T ' D SRC+ZO.ZbTHQ
55 56
I
0.46 0.45 0.51 0.56 0.69 0.69
4
0.20
..>
I
0.10
0.2
0.0
1
0 REACTION T I M E ,
1
,
t
I
I
I
I
I
I
I
10
20
30
40
50
60
70
REACTION TIME,
MINUTES
Figure 5. Effect of the addition of THQ to SRC-I1 solvent on the conversion to pentane solubles. 1.0
I
-
W
I
I
I
I
I
I
+
