IGnetics of Coal Hydrogenation CONVERSION OF ASPHALT SOL WELLER, M. G, PELIPETZ, AND SAM FRIEDMAN U. S . Bureau of Mines, Bruceton, Pa, after vacuum stripping the autoclave. (Vacuum stripping was carried out at 100' C.) The two results usually agreed within experimental error (0.3 to 0.5 gram); because the values obtained by oxygen balance were somewhat more consistent, they are given in Table I. After the vacuum stripping the autoclave residue was extracted a t room temperature with 400 ml. of benzene; the slurry was centrifu ed, and the centrifuge residue was exhaustively extracted wit% benzene in a Soxhlet apparatus. The residue in the Soxhlet thimble, after drying, constituted the benzeneinsoluble material. The total benzene extract was freed of benzene by distillation; the residue was then treated with about five times its own weight of n-hexane. After the mixture had stood 16 hours, the hexane-insoluble material was filtered off, washed with two 75-ml. portions of hexane, and dried. This benzene-soluble, hexane-insoluble material is defined as asphalt. The hexane was then distilled from the total hexane extract; the residue (heavy oil), plus the oil which was removed in the vacuum stripping operation (light oil), constituted the total oil fraction.
Because asphalt seems to be an intermediate product in coal hydrogenation, the study of its hydrogenation is helpful for quantitative evaluation of the over-all coal hydrogenation reaction and for developing improved coal hydrogenation processes. The conversion of asphalt is first-order with respect to asphalt remaining and apparently also with respect to hydrogen pressure. The average activation energy in the range 430" to 440" C. is 36 1cg.-cal. per mole. Of the asphalt converted, about 92% goes into the production of oil and 8% into hydrocarbon gas over the entire temperature range studied (400' to 440"C.), Taken in conjunction with data on whole coal, the results of this study permit a quantitative calculation of the amount of asphalt present during coal hydrogenation f o r a range of temperatures and times.
A
SPHALT, which may be defined as material soluble in benzene but insoluble in n-hexane, is almays formed during coal hydrogenation, and it s e e m to be an intermediate product in the over-all conversion of coal to distillable oil (3-6). Study of the kinetics of asphalt hydrogenation is of interest for two reasons: (1) The presence of asphalt in coal hydrogenation products ia undesirable, and knowledge of conditions for eliminating asphalt would be helpful in developing improved coal hydrogenation processes; ( 2 ) if asphalt is an intermediate in the hydrogenation of coal, determination of the kinetics of asphalt hydrogenation is essential for quantitative evaluation of the over-all coal hydrogenation reaction. Isolation of a large quantity of coal hydrogenation asphalt from the products of a pilot plant run has permitted the investigation of asphalt hydrogenation to be carried out directly. All the experiments described were performed in batch autoclaves. The variables studied were temperature, time, and, to a limited extent, pressure. Application of the results on asphalt to the kinetics of coal hydrogenation will be made in a subsequent paper.
The quantities of gaseous hydrocarbons produced were determined both by mass spectrometer analysis of the gas bled from the autoclave (method I) and by a method (method 11) involving weighing the entire autoclave plus contents on a bullion balance before and after charging, and before and after bleeding off the gases. In method I1 the hydrocarbon gases were calculated by a method of successive approximations from the hydrogen balance, the weight of hydrogen charged, and the gasification loss of the coal charge. A first approximation to the hydrocarbon gas production is obtained either from the gas analysis or from the carbon balance. As the hydrocarbon gas composition is essentially constant in all experiments (corresponding to an average hydrocarbon molec-
EXPERIMENTAL
The crude asphalt used in these studies was isolated from the products of a pilot plant run on the hydrogenation of Bruceton (Pittsburgh-seam) coal. "Heavy oil letdown" (which contains the high-boiling liquid products, ash, catalyst, and unreacted coal) obtained from the pilot plant was treated with eight times its weight of n-hexane. The insoluble material was filtered, washed with n-hexane, and then treated with one and onehalf times its weight of benzene. The slurry was filtered; the filtrate, after removal of the benzene by distillation, constituted the crude asphalt used. By laboratory analysis (see below) this asphalt was found to contain 93.8y0 asphalt (benzenesoluble, hexane-insoluble), 4.4% oil (hexane-soluble), 0.4% water, and 1.4% benzene-insoluble material; the ash content was 0.03010. The ultimate analysis of the crude asphalt is shown in Table IV. The apparatus ufied in t,he hydrogenations has been described ( 8 ) . Initial cold hydrogen pressures of 1250 or 2500 pounds per square inch gage were used, The rotating autoclaves were heated to reaction temperature over a period of about 80 minutes, held a t tem erature for the desired length of time, and then cooled; cooing to below 300' C . required about 30 minutes. The water production was determined by an oxygen balanre as well as by measurement of the water in the overhead product
TABLE1. PRODUCT DISTRIBUTION FOR ASPHALT HYDROGENATION
Temp., C. 400 400 400
(2500 pounds per square inch initial Time a t Reaction Temp., InsoluTotal Min. bles .4rphalt Oil
400 400
0 30 60 120 180
3.0 0.9 0.5 0.5 0.4
86.0 84.8 78.8 77.8 70.4
8.7 11.3 17.5 18.7 26.6
430 430 430 430 430
0 30 60 120 180
0.7 0.6 0.5 0.5 0.6
65.5 49.3 29.9 17.2
87.7
10.4 30.6 44.0 61.0
440 440 440 440 440
0 30 60 120 180
0.3
0
,,,
...
...
0.4
81.6 62.1 43.1 18.3 8.6
hydrogen pressure)
2.2
3.6 5.9
7.0
72.7
8.3
16.1 31.6 50.1 73.8 79.2
1.9 3.6
5.7
8.5 12.3
1.1 1.8 2.2 2.3 2.0
1.20 1.90 2.32 2.76 2.88
1.6 2.1 2.1 2.6 2.3
1.10 1.72 2.16 2 40 3.62
Obtained by oxygen balance, oorrected for initial moisture.
b 1250 lb./aq. inch initial hydrogen pressure.
1572
He
HydroConsumed carbon % of .4sphht Gases Water" Charged 1.8 1.1 0.72 2.2 1.4 1.32 2.6 1.6 1.62 3.1 1.9 1 74 3.2 2.1 2.04
July 1951
1573
INDUSTRIAL A N D ENGINEERING CHEMISTRY
ular weight of 30), this permits a first approximation to the hydrogen consumption t o be obtained from the hydro en balance. From the relation that the weight of hydrocar%on gas produced equals the observed weight loss of the charge plus the weight of hydrogen absorbed, a second ap roximation to the hydrocarbon gas production is obtained. $his process is continued until consecutive values of the hydrocarbon gas production are identical; usually two or three iterations are sufficient.
-
I‘ 0
8
160
w
a 5
150
r
% a
140
3 (D
L
130
W
3 I20
1.00 l
,
1
(2)
I
I
c
.90 0
where f ( p 3 is some function of the hydrogen partial pressure and k’ = k j ( p ~ , )and ,
I
I80
RESULTS AND DISCUSSION
Quantitative treatment of kinetic data obtained in batch autoclave experiments is difficult, especially when the heating up and cooling periods are of the same order of magnitude as the periods at the desired reaction temperature. (In the following discussion the heating up and cooling periods are lumped together and referred to as the heating u p period.) The problem of determining the progress of a reaction a t a given temperature would be minimized if samples could be taken a t intervals during the course of a single experiment. Unfortunately, this is frequently impracticable for high-temperature, high-pressure reactions, and it is usually necessary to employ each experiment to determine a single point rather than a complete curve. The question of the heating up period is then most easily resolved by assuming, as a f i s t approximation, that all reactions occurring have about the same activation energy. If this assumption is made, the heating up period is then equivalent to a short period at reaction temperature. If each experiment involves the same heating up period, experiments for different times a t the same temperature can be directly compared, and the correction for the heating up period is simply a shift in the zero time for the reaction-that is, the system behaves as though there were no heating up period, and the “true” reaction time exceeds the nominal time at temperature by a small constant amount. Because several activation energies are in general involved, however, this method of handling the heating up period is inexact, and the error involved should be minimized by using only one heating up and cooling period in a single experiment (a precaution not always observed in work of this kind). The distribution of products obtained in each hydrogenation experiment is summarized in Table I. Also included in this table are the hydrogen consumptions, expressed as weight per cent of asphalt charged. No appreciable amounts of carbon monoxide or carbon dioxide were observed, Figure 1 contains the semilogarithmic plots of asphalt remaining versus nominal reaction time a t temperature. Within experimental error, a straight line is obtained for each temperature. The conversion of asphalt is, therefore, first-order with respect to asphalt. If A represents the asphalt remaining,
I
I90
An independent check of method I1 is possible by comparing the calculated and observed weights of the total gases bled off; these checks were always excellent. The values for hydrocarbon gas production shown in Table I are those calculated by this method. They were almost invariably more self-consistent than those obtained from the mass spectrometer analyses, The charge in each experiment was 50 grams of asphalt, 0.5 gram of stannous sulfide, 0.25 gram of ammonium chloride, and 14 grams of hydrogen (2500 pounds per square inch initial pressure), or 7 grams of hydrogen (1250 pounds per square inch initial pressure). The fraction of the hydrogen charge consumed by reaction was small, exceeding 10% in only a few cases; the hydrogen partial pressure was essentially constant, therefore, for all experiments carried out a t the same initial pressure.
A / A o = e-k’t
I
ZOO(
30
I
60
90
0
1
120
150
l
180
1
210
NOMINAL TIME, MINUTES
Figure 1.
Rates of Asphalt Hydrogenation
A , being the amount of asphalt originally present. The lines shown in Figure 1 were obtained by the method of least squares. The specific rate constants, k’, may be oalculated from the slope8 of the lines. Values of k’, as well as their probable errors ( I ) , are listed in Table 11. The specific rate constants show a large temperature dependence. Figure 2, which is a plot of In 10%’ us. l/!!’ for t h e hydrogenations a t 2500 pounds per square inch initial pressure, shows this dependence. The vertical line passing through each point indicates the magnitude of the probable error of the rate constant. It is clear that the simple Arrhenius relation 8’ =
(- gj) does not hold over the temperature
Iclexp
range
studied. Application of this formula to the results a t 430’ and 440°,for which the greatest accuracy in k‘ was obtained, leads to a value of 35.8 kg.-cal. per mole for the activation energy of asphalt conversion. Even higher values of the activation energy would be calculated from the data at lower temperatures. The large dependence on temperature exhibited by the reaction rate implies that any ratedetermining step involves a chemical reaction and not, for example, the diffusion of hydrogen through a boundary film. The variation of effective activation energy with temperature may be taken as prima facie evidence that the overall reaction is a composite one.
TABLE11. Temp., 0
c.
400 420 430 440 430
RAT^ CONSTANTS FOR ASPHALT HYDROGENATION Initial p ~ n Lb./Bq. Inch &age 2500 2500 2500 2500 1250
k’, Min.-1 0.00107 * 0.00011 0.00503 * 0,00036
0.00895 =t0,00009 0.01282 * 0.00029 0.00440 * 0.00030
The dependence of rate on hydrogen pressure was determined a t only one temperature, 430°, at which temperature the reaction proceeds a t a rate convenient for measurement. As indicated by Figure 1 and Table 11,within the pressure range studied and within experimental error, the specific rate constant is approximately linear with hydrogen pressure. Therefore, k j ( p ~ ~ ) E k‘ = PHI), orj(pa2) = pHa, and Equation 1is to be written
INDUSTRIAL AND ENGINEERING CHEMISTRY
1574
Vol. 43, No. 7
than that shown by Figure 3. (This is, ])orlittps, t o cspected, twcauee t h e quantity of gas n-as usually small and subject to relatively greater error in its determination.) The slope of t,ho liiw coiresponcis t o a value of 0.088 for k c l k ' . Of the asphalt re:trting, t.herefore, approximately 8.8'30 is converted to hytlrocar-
bon gas. L3timate analyses of t,he light and heavy oil fraotioi~swere essentially identical in all experiments. hverage a i i a i y w arc shown in Table 111. Ultimate analyses of the residual asphalt fractioiiP arc shown iri T:tbie IV. At each temperature thcre is a email, coiitinuous shift, in composition vitli increasing reaction time. The :in:ilyses
r.
1ARI.E 111. ELEXEKTARY Comowrrorzs OF I,IGHT A X ] ) ~II.;A\-Y OIL ~ ~ _ Weight ________ Fiaction Light oil Heary oil
r .
1 A B l X Iv.
'Yeiiir),, ' C.
The initi:ti hydrogen pressurcs of 1250 and 2500 p(iunds p ~ r square inch correspond to pressures a t reaction te~iipcrat~ure oi' about 3000 arid 6000 pounds per squarca inch, respectively. This is in the norinal r:irige lor large scale ccial or titr hytlrogtwa~~ioii. The linear pressure dependence in this range enipihltsizes thr. feeling held by workers in the field of coal hydrogenation that the necessity for high hydrogen pressure. is of kinetic, and riot t h t r modyriarnic, origin. If one assumes that the asphalt hydrogeiiation proceeds with the simultaneous production of oil and of hydrocarbon gas, each of these reactions being first-order with respect t o asphalt, reniaiiiiiig, one may write (at conPttirit hydrogen pressure)
0
C
H
S
84.58
11.93 8.89
0.45 0.57
88.73
ELE\fESTARY
COSfPOSI'rIONS
6 0.09 0.13 OF
(by diff.)
3.0 1.7
ORIGIS.\L
AND
RESIIILTAL ASPHALTS
Time at Reaction Temp., Nin.
C
1-1
1-
Original nsphalt
88.34
S.44
1.91
0.11
3.2
400 400 400 400 $00
0 30 60 120 180
88 68 88.72 88.81 89.47 89.61
6.90 7.21 7.26 7.32 7.47
1.87 1.35 1 44 1.26 1.11
0.22 0.20 0.24 0.24 0.18
2.5 2,5 2.3 1.7 2.1
420 420 420 420
0 30 60 120 180
89.10 89.10 89.38 89.86 89,56
7 04 7 43 7.50 6.98 6 86
L.4,i 1,lG 1.00 1.04 0 . $17
0.19 0 14 0.15 0.12 0.12
2.2 2.2 1.9 2 0 2 5
430 430 430 430 430
0 30 60 120 180
88.97 89.31 89.82 90.32
6 .99 7.28 7.12 0.96 6.75
I .43 1 23 1.11 0.57 0.17
022 0.18 0.19 0.16 0.18
2 4 2.0 1.8 2.0 2.5
440 410
0 30 60 120 180
89.15 90.13 89 82 91.11 91 3:
7.28
1.34 1.03 1 27 0.96 0 92
0.17 0,li 0.14 0.17 0 13
2.;
0
89.19 89.47 89.73 90.57 90.45
8.87 6 95 7.07 6 72 6.38
1 47
0.18 0.14 0.21 0 00
2.3
0.11
1. 8
-420
440 440 4.10 430" 430" 430" 430a 430n
30 60 120 180
90,44
7.17. 0.75 6 27 3 78
1 29 I .2i
1 12 1 .os
l.J
2.0 1.5 1 6
:1.; 3 ,
~ l i e r c0 and G tire the quantities of total oil aiid hydrocarbon gas, i,espectively, at anv time. Integration of Equation 4 leads to
0
=
k +lo
-
-4)
+ 'L4
(6)
t h e liist limri arising Iw:iuw the crude asphalt initially coiitainetl 4,-$yo oi' oil. I'igui'e 3 is R plot of oil versus asphalt, remaining for all c:f thr: csperinients :it 2500 pounds per squ:ii.e inch. i\ straight line is oht:iiiicd, in tigreenlent x i t h Equation 6. 1;:valu~~tion of the ciat:t by the method oi least square^ shn\w .I,= 92.8%, iii reasoii:rl,lc agreenient xith the obse 93.8%, and k,l/li.' = G.912. . This latter figure nimiis that, 91 ,270 o f the asphalt \r.hic:h is hytlrogenated is csonverted t o oil. Of cspecial interest is the. fact that, within u.;periiinelii,lil e r r o ~ ~ , puints for each of the four temperatures investigated lie along the saiiie ztraight line--that is, the fraction of reacted asphalt that is converted t o oil is constant within thc tcxmper:iture region
studied. Integration of Equation 5 leads to a relation similar t o Equation 6, predicting a straight-line relationship between hydrocarbon gas and asphs!t remaining. This hchavior is espcrinientallg ol)sctrvtd, hut, the wtittvr ~,t tlica p)inti is conuicleralily greatpi
ASPHALT REMAlh NG, PERCENT
Figure 3.
Oil Production c s . i ~ p h a l tCtsniersion
July 1951
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
illdieate either that the asphalt undergoes a slight hydrogenation initially, which is followed by a partial dehydrogenation, particularly a t higher temperatures, or, alternatively, that the variations in hydrogen content reflect the heterogeneity of the asphalt fraction, some-constituents being more easily hydrogenated and split than others. ACKNOWLEDGMENT
The authors are grateful to R. A. Friedel for furnishing the and to Joseph Lederer Of gas ‘pectrometer for the ultimate analyses of products. LITERATURE CITED
(1) Davies, 0. L., “Statistical Methods in Research and Produc-
tion,” pp. 152, 252, London, Oliver and Boyd, 1949.
1525
(2) Fisher, C. H., Sprunk, G . C . , Eisner, A., O’Donnell, H. J., Clarke, L., and Storch, H. H., Bur. Mines Tech. Paper 642
(1942). (3) Kata, M., and Glenn,
R.9., Division of Gas and Fuel Chenllmtry, 115th Meeting, AM. CHEM.SOC., San Francisco, Calif., 1949. (4) Pelipetz, M. G., Kuhn, >I., Friedman, S., and Storch, H. H., IND.ENG.CHEM.,40, 1259 (1948). ( 5 ) Weller, S.,Clark, E. L., and Pelipetz, M. G., I b i d . , 42, 334 (1950). RECEIVEDMarch 1, 1950. Presented before t h e Division of Gas a n d F u e l Chemistry, Symposium on Kinetics of Coal Hydrogenation, a t t h e 118th Meeting of t h e AMIBRICAA CHEMICAL. SOCIETY,Chicago, Ill. For material supplementary to this article (tables showing elemental balances for all the hydrogenation runs) order Document 3117 from the American Documentation Institute, 1719 N St., N.W., Washington 6, D. C., remitting $1 for microfilm (images 1 inch high on standard 35-mm. motion picture film) or $1 for photocopies (6 X 8 inches) readable without optical aid.
(Kinetics of Coal Hydrogenation)
CONVERSION OF ANTHRAXYLON SOL WELLER, M. G. PELIPETZ, AND SAM FRIEDMAN U . S. Bureau of Mines, Rruceton, Pa.
T h i s study was undertaken in an effort to acquire additional information on the reaction mechanism of coal hydrogenation, and, in particular, to determine whether asphaltene (material soluble in benzene but insoluble in n-hexane) is an intermediate in the conversion of coal to distillable oil. A batch autoclave stud)- was made of the kinetics of hydrogenolysis of bituminous coal anthraxylon. The range of conditions included conversion of coal to a yield of 8Oa/, asphaltene and 8qo oil at one extreme, and a yield of 6qo asphaltene and 76Yo oil at the other. The conversion
of coal to oil can be quantitatively described by the reaction scheme, coal +asphaltene -oil, both reactions being of first order, kinetically. Water and gas are by-products of both reactions. The results furnish eiidence that asphaltene is an intermediate product in coal hydrogenation, and provide quantitative data on the rates of the coal-to-asphaltene and asphaltene-to-oil reactions. This information will be useful in understanding the nature of the coal hydrogenation reactions and, perhaps, in permitting more ratimal design of practical processes for the over-all conversion of coal.
P
gen, 0.8% sulfur, 7.6% oxvgen (by difference), 1.4% ash, and 1.4y0 moisture. The pure asphalt previously studied ( 3 ) was isolated from a pilot plant run in which coal from the same mine was hydrogenated; the results of the experiments with purr asphalt should, therefore, be applicable t o those with the parent coal reported here. The experimental procedures and methods of calculation wcr(identical with those given in detail in previous papers ( 3 , 4). The charge in each experiment was 50 grams of anthraxylon (48.6 grams of moisture- and ash-free coal), 0.5 gram of stannous sulfide, and 0.25 gram of ammonium chloride. No oil vehicle was used. The initial cold hydrogen pressure was always 2500 pounds per square inch gage. In no experiment did the hydro en consumption exceed 177, of the initial hydrogen charge, so t k a t the experiments can be considered to have been made a t essentially constant pressure. The term “liquefaction” employed subsequently in this paper is defined as 100 minus per cent organic benzene-insolubles.
REVIOUS data ( 1 ) have indicated that, asphalt may be a n intermediate product in the over-all hydrogenolysis of coal to distillable oil. (The term “asphalt” is used here to designate material soluble in benzene but insoluble in n-hexane.) To demonstrate the validity of this hypothesis and t o gain additional information about the reaction mechanism, a batch autoclave study has been made of the kinetics of the hydrogenolysis of anthraxylon. -4s a preliminary to this investigation, the kinetics of the hydrogenolysis of pure asphalt were studied (3). It has been found possible to interpret the data on coal hydrogenolysis quantitatively in terms of successive first-order reactions involving asphalt as an intermediate, no additional assumptions being made concerning the rate of asphalt hydrogenolysis. Some earlier Bureau of Mines n-ork on the kinetics of coal hydrogenolysis (8) differed from that reported here in a t least three major respects: In the earlier experiments, Tetralin was employed as a vehicle; the initial hydrogen pressure was 1000 pounds per square inch gage; and extended runs were carried out in stages, between which the autoclaves were cooled to room temperature and recharged with hydrogen, EXPERIMENTAL
I n order t o minimize complications arising from the heterogeneity of coal substance, hand-picked anthraxylon was used in this study. The material was picked from the experimental mine of the Bureau of Mines a t Bruceton, Pa., and it consisted of Pittsburgh-sear,. bituminous coal of the following composition, as used: Ll.Syo carbon, 5.35y0 hydrogen, 1.6% nitro-
RESULTS AND DISCUSSION
The product distributions and hydrogen consumptions observed in these experiments are summarized in Table I. T h e values for “oil” pertain to liquid product which is soluble i n 7 ~ hexane. “Water” includes only water formed from the organic material; it is corrected for the moisture originally present in t h o coal. Within experimental error, the conversion of coal to liquid and gaseous products a t 400’ C. is first-order in coal remaining. (Asphalt is included with the liquid products.) This is shown in Figure 1, which is a semilogarithmic plot of organic benzene.. insolubles, corrected for refractory material (see below), versus
