Hydrogenation of a Pittsburgh Seam Coal
cooled t,o room temperature a t end of each 3-hour period and recharaed - with hvdroaenl was employed. The products obtained in experiments 355-3, 340-9, 310-3, and 310-9 were too viscous to be centrifuged directly, as was previously done. Instead, the solid and semisolids were washed with acetone repeatedly at room temperature and centrifuged to separate the residue and acetone washings. The effect of this necessary variation in procedure on the yield of acetone-insoluble residues (from which the liquefaction yields were calculated) is not known. It might be stated in this connection that adding acetone to the ash-free tetrahydronaphthalene solutions obtained by centrifuging the fluid hydrogenation products of other experiments (made at 370" C. and higher) caused precipitates t o appear that were not completely redissolved by extraction with acetone and benzene, The acetone washings and the centrifuged of oils were distilled to obtain the residues (pitches substantially free of ash) and distillates described in Tables IV and VIII. The distillation residues were brittle solids or viscous oils, depending upon the conditions of hydrogenation. Since the experiments reported in this paper were made over a period of many months with a coal of comparatively low H. H. STORCH, C. H. FISHER, ABNER EISNER, rank. the coal was analyzed later to determine the extent of weathering and oxidation. AND LOYAL CLARKE The results (Table I) show that some oxidation had occurred. Most of the hydrogenaCentral Experiment Station, U. S. Bureau of Mines, tion experiments were made before the coal Pittsburgh, Penna. had reached the stage of oxidation indicated in Table I. It is likely that the oxygen added to the coal on standing is easily removed by hydrogenation. Since the oxygen remaining in the pitch and insoluble residue was used as the basis for calculating the oxygen eliminated, the LTHOUGH i t is well known that the primary liqueoxidation occurring before hydrogenation should have little effect faction of coal by hydrogenation (6) is accompanied by on the accuracy of the oxygen data. All the oxygen determinathe removal of considerable quantities of oxygen, tions were made by difference and hence include the usual analytical errors. Bruceton coal (3) contains 63 per cent anprinciparly as water (as well as the elimination of nitrogen and thraxylon, 22 per cent translucent attritus, 12 per cent opaque sulfur), there are not enough published data to show the effect attritus, and 3 per cent fusain. of physical conditions on its removal and to indicate the relations between the rates of hydrogen consumption, oxygen Rate of Hydrogen Consumption elimination, and liquefaction. English investigators ( 2 ) found that most of the oxygen of Figure 1 shows the rate a t which hydrogen is consumed a t the coal was eliminated in the early stages of hydrogenation various temperatures. After a steep rise during the first hour and believed that a considerable portion of the oxygen rethe curves of hydrogen consumption us. time are a series of moved was present in the coal as hydroxyl groups. Fisher virtually straight lines whose slopes show a temperature coand Eisner (1) showed that about 60 per cent of the oxygen efficient of about 1.2 for a 10" C. temperature change. This in Pittsburgh bed coal (from the bureau's experimental mine rather low temperature coefficient indicates that the slow step at Bruceton, Penna.) was easily and rapidly eliminated and involved in the rate of hydrogen consumption is probably a that the remaining 40 per cent was removed with greater diffusion process. The change in slope of the 415' C. curve difficulty. On this basis they suggested that the oxygen exat about 9 hours may be due to reversible hydrogenation of isted in the coal as two main types, but they did not speculate unsaturated compounds, the equilibrium favoring saturation as to the specific nature of the oxygen groupings. The present a t 400" C. but changing rapidly a t higher temperatures in paper gives additional data on the elimination of oxygen from favor of the unsaturated molecules. This statement is also Bruceton coal by destructive hydrogenation and points out supported by Figure 12, which shows that the maximum hyseveral relations between rates of hydrogen consumption, drogen content of the main product of the reaction (pitch) oxygen removal, and liquefaction. occurs at about 410" C. The steep rise during the first hour -
I
Kinetics Hydrogen Consumption, Oxygen Removal, and Liquefaction
A
for all of the curves of Figure 1 may be due to the initial solu-
Hydrogenation Procedure The coal, equipment, and procedure previously described (I), were used: Bruceton coal (100 grams), ground to pass 200mesh, 100 grams of tetrahydronaphthalene, and 1 gram of stannous sulfide were placed in a 1200-cc. bomb; after it had been flushed with nitrogen and hydrogen, hydrogen was added until the pressure at 20" C. was 1000 pounds per square inch (70.3 kg. .per sq. cm.). The time and temperatures are given in Table 11. In the longer experiments the stepwise method of hydrogenation (converter
OF FRESHLY PREPARED AND STORED TABLE I. ANALYSES BRUCETON COAL
Condition
prepared After standing 13 mo.
346
AsReceived M oisture Ash
c
H
Dry, Ash-Free Basis Volatile C/H matter ratio
-
C U 0 S Per cent b y weight
Calorific
vaiue G. cal.
1.6
6.3 5 . 6 84.2 1.7 6.8 1.7
38.8
15.0
8361
1.4
6 . 3 5 . 5 8 3 . 4 1 . 6 7 . 8 1.7
38.4
15.2
8283
INDUSTRIAL AND ENGINEERING CHEMISTRY
MARCH, 1940
347
56
v)
I
48
s
U
640 W
5
v)
g32 0
b E24
eI
1.6
TIME HOURS
6
10
20
40 50 60 70 LIQUEFACTION YIELD, PERCENT
30
80
90
YIELDSAND HYDROQEN FIGURE 4. LIQUEFACTION CONSUMPTION v)
24 U
Ei
s
4 z p 2
3
0
A 8 12 TIME OF HYDROGENATION, HOURS
tion of hydrogen in the liquid phase necessary to reach the steady state involved in the diffusion process. The latter may be the diffusion of dissolved hydrogen through a liquid film covering the catalyst surfaces. The existence of a greater proportion of unsaturated molecules at temperatures higher than 400" C. favors condensation reactions of various kinds; these reactions may result in the formation of insoluble compounds that are further dehydrogenated and condensed at higher temperatures (above about 440" C.) to form coke. The use of pressures higher than those employed in obtaining the data of Figure 1 would probably
increase the temperature a t \bhich the abrupt change in slope of the hydrogen consumption rate curves is first observed. The curves of Figure 1 are characteristically different from those for the rates of oxygen elimination and of coal liquefaction, as given in Figures 2 and 3, respectively. I n Figure 2 the rather sharp change in slope of all of the curves a t about 60 per cent removal of oxygen and in Figure 3 similar changes in slope between 80 and 90 per cent liquefaction constitute the chief differences. These comparisons show that there is no direct or simple relation between the hydrogen consumption rate and the rates of oxygen removal and of coal liquefaction. This conclusion is supported further by the data in Figures 4 and 5 . I n Figure 4 the liquefaction yields are plotted against the hydrogen consumed. Below 80 per cent liquefaction the curves are virtually straight lines parallel to the per cent-liquefaction axis; thus they indicate large changes in per cent liquefaction with only a slight change in hydrogen consumption. In the later stages beyond about 80 per cent liquefaction there is a sharp increase in hydrogen consumption with only little change in the degree of liquefaction, which indicates that the principal reaction in this region is that of hydrogen with the liquid phase. Figure 5 , where hydrogen consumption is plotted against oxygen elimination, shows that little hydrogen is used to remove oxygen in the early stages-that is, up to about 60 per cent oxygen elimination. Similar lack of dependence of the carbon content of the main liquefaction product (pitch) upon hydrogen consumption
FIGURE5. HYDROGEN CONSUMED A N D OXYGEN REMAINING IN RESIDUE AND PITCH
348
INDUSTRIAL AZND ENGINEERING CHEMISTRk-
VOL. 32. NO. 3
I n Figure 7 the oxygen-removal rates are plotted as if the total reaction were first order with respect to the concentration of the coal substance-that is,
+
TI
t = IC log (a - 2) k' here t = time a = initial oxygen concentration in coal substance ?: = amount of oxygen - - removed from coal substance at time t k , k' = constants representing slope of curve and intercept on log (a - x) axis, respectively
For a siinple unimolecular reaction these curves mould be straight lines. The curves of Figure 7 indicate that the reaction IS either one of order higher than first or consists of a number of consecutive reactions. The data were also plotted accorcling to the bimolecular law for a single reactant, t =
X
ka(a
- 2)
The initial slopes of these curves should be proportional to the reciprocal of the rate constant, k , and hence the slopes should decrease with increasing temperature. Since the reverse of this was true, it was decided that the bimolecular treatment was not the correct one for this reaction. FIGURE6. HYDROGEN CONSUMPTION AND CARBOX CONTENT OF PITCHES
is shown in Figure 6. Since one of the main factors in increasing the carbon content is elimination of oxygen, the abrupt changes in slope of the curves of Figure 6 might have been predicted from Figure 5 . The variations in slope of the almost vertical portions of the curves of Figure 6, corresponding to the regions in which hydrogen consumption is increasing rapidly with small changes in carbon content, may be significant. A possible interpretation is as follows: At 385" and 430" C. (where the curves are practically vertical), approximately enough hydrogen was absorbed to compensate for loss of oxygen, nitrogen, and sulfur. Presumably this hydrogen reacted mainly to saturate double bonds a t 385" C.and to cleave carbon-to-carbon bonds at 430" C.
Kinetics of Oxygen-Removal Reactions Data for the rates of oxygen removal from Bruceton coal b y hydrogenation a t various temperatures are showi in Figure 2. I n considering these rates it should be recalled that they are independent of the rates of hydrogen consumption. Since the slowest step in the hydrogen-consumption mechanism is probably diffusion through a liquid film on a catalyst surface and since, as will be shown, the oxygen-removal reactions have temperature coefficients considerably larger than 1.2 per 10" C., i t is equally probable that utilization of hydrogen a t the catalyst surfaces is not the slowest step in the oxygen-elimination reactions. Hence it is also probable that the oxygenelimination reactions are independent of the hydrogen concentration within the limits of variation of this factor iii our experiments. The oxygen-removal reactions therefore depend only upon the coal-substance concentration, or they are zeroorder catalytic reactions or a combination of both of these reaction types. In considering Figure 2, one ib tempted to say a t once that the rather abrupt changes in slope of these curves a t about 60 per cent oxygen removal indicates a rapid first-order thermal reaction followed by a zero-order catalytic process obscured at temperatures higher than 400" C. by some secondary reaction that retards further elimination of oxygen. However, it is possible that a series of successive unimolecular reactions might give the types of curves shown in Figure 2 .
FIGCRE 7 . OXYGEN-REMOVAL DATAPRESENTED
AS
FIRST-
ORDER RE.4CTION
X h e n corrected for the 2" C. per minute rate of heating to reaction temperature, the initial slopes (at t = 30 minutes) of the curves of Figure 7 yield the following temperature coefficients : Temp. interval, C. Temp. coefficient per 15' C. Activation energy, kg. cal.
370-385 3.4 69
385-400
2.9
65
400-415
2.8 64
;It t = 8 and 12 hours, the temperature coefficients are as fOll0.s~' Temp. interval, C. Temp. coefficient per 16' C. 1 8 hours t = 12 hours Activation energy, kg. oal.
-
370-385
-2.0 1.1 ?
385-400
2.0 2.0
42
400-415 -1.7 -1.5 7
The reproducibility of the data of Figure 7 may be gaged from the three points a t t = 1 hour on the 400" C. curve, the two points a t t = 6 hours on the 385" C. curve, and the points at t = 8 and 9 hours on the 413" C. curve. This reproducibility is such that, although the slopes of the curves may be
MARCH, 1940
INDUSTRIAL AZiD ENGIKEERING CHEMISTRY
altered considerably, the general picture of the kinetics of the reaction as developed will not change much. The temperature coefficients appear to show that the initial reaction for temperatures above 370" C. is a decomposition of the coal substance involving the elimination of oxygen. The activation energy of this reaction is about 65 kg. calories. Subsequently the orer-all reaction (except for the 385-400" C. interval) has a negative temperature coefficient. The negative temperature coefficient for the 370-385" C. interval may be due to the fact that a t 370" C. the initial reaction has not yet been completed a t 8 hours reaction time; hence this comparison of rates is not valid. Between 385" and 400" C. the positive temperature coefficient of a second reaction or group of reactions appears. Thwe reactions may be catalytic in nature, involving the adsorption of the reactants on a solid surface. Between 400" and 415' C. this second reaction is largely obscured by condensation reactions which result in the production of molecules more stable than the original coal substance and from which oxygen is eliminated a t a very slow rate.
349
TABLE 11. LIQUEFACTION YIELDD.~TA
Temp.
Time
C. 44.5
Hr.
430 430 415 400 400 385 370 370 370 370 370
3 3 6 15
-ProductsLiquids, solids -Grams----169.6 184.3 179.9 174.4 18SC 181.6
--Conversion ofPure Coalb Based on Centri- Based on residue fuged residue calcd. from Oil" found ash content
Gases
Loss
%
%
%
19.3
15.9
76
85.8
11.8 13.1 11.6C 11.5 6.3 10.4 8.6 10.2 7.5 13.3
80 82 31 84 83 22 35 82
92.: 93.3
82.9 90.3 91.9
8.9 14.1 20.0 2.8 13.9 10.8 1.6 2.8 4.3 5.2
10.7
..
92.8
93.4
74.6 69.6 15 93.3 15 188.6 9f.b 92.2 3 190.1 ... 45.5 6 191.3 73.8 71.8 9 189 90.1 89.6 12 191.5 78 90.2 89.6 15 185.5 5.9 .. 90.7 89.6 355 3 196.2 1.0 4.7 .. ... 33.1 355 9 186.3 2 . 7 C 14.6C 58 ... 81.6 340 9 195C 2.5C , .. .. 50.9 48.3 310 3 191 0.9 9.9 .. 26.3 14.7 310 9 185 2 . l e 15.8C ,, 23.8 19.8 0 Per cent of material centrifuged. b Per cent of dry, ash-free coal converted into gases, liquids, and soluble material. C Approximate, 1
Rate of Liquefaction Liquefaction was measured by determining the amount of residue obtained upon centrifuging the tetrahydronaphthalene solution of the product and subsequently washing the centrifuge residue with acetone and benzene. It is apparent therefore that the term "liquefaction" means decrease in molecular weight until the products are gaseous under normal conditions or dissolved by the treatment that has been described Any further decrease in molecular weight will not be evaluated by the percentage liquefaction. Figure 3 and Table I1 show the percentage conversion of the dry, ash-free coal into soluble material plus gases a t various times and temperatures. Since the extent of liquefaction is independent of the rate of consumption of hydrogen and since, in most instances, virtually all of the tetrahydronaphthalene was recovered as such, the rate of liquefaction may be considered to depend only upon the concentration of the coal Pubstance. A plot of the data using the equation for a first-
FIGURE8. LIQUEFACTION DATAPRESENTED .4s FIRSTORDERREACTIOK order reaction is given in Figure 8. When corrected for the 2" C. per minute rate of heating to reaction temperature, the initial slopes (at t = 30 minutes) yield the following temperature coefficients: Temp. interval C. 310-355 Temp. coefficieht per I 5 O C. 1.5 Activation energy, kg. cal. 6.6
355-370 1 8 32.0
370-385
2.7
56.0
385-400 400-415 2 9 2.7 65 0 62.0
At 8 and 12 hours the temperature coefficient
