INDUSTRIAL AND ENGINEERING CHEMISTRY
820
Kolthoff,I. M., Lee, T. S., and Cam, C. W., J . Polymer Sci., 1, 429 (1946).
Kovacio, P., Rubber Age, N . Y., 75, 700 (1954). Langdon, R. G., and Bloch, K., J. Biol. Chem., 200, 129 (1953). Leeper, H. M., and Schlesinger, W., Science, 120, 185 (1954). Mark, H., Textile Research J., 23, 294 (1953). Marvel, C. S., and Kraiman, E. A., J . Org. Chem., 18, 707 (1953).
Mayo, F. R., and Walling, C., Chem. Reva., 46, 191 (1950); Alfrey, T., Bohrer, J. J., and Mark, H., “Copolymerization,” Interscience, New York, 1952. Mochel, W. E., and Peterson, J. H., J . Am. Chem. Soc., 71, 1426 (1 949)
I
Morton, A. A., Rubber A g e , N . Y., 72, 473 (1953). Peters, R., Endeavour, 13, 147 (1954). Potter, V. R., and Heidelberger, C., Nature, 164, 180 (1949); Rudney, H., Lorber, B., Utter, M. F., and Cook, M., Federation Proc., 9 , 179 (1950). Resing, W. L., India Rubber World, 130, 670 (1954). Rittenberg, D., and Bloch, K., J . Biol. Chem., 154, 311 (1944). Ritter, F. J., India-Rubber J., 126, 55, 70 (1954). Robinson, R., I X Congr. intern. qutm. pura 2/ aplicada ( M a d r i d ) , 5, 17-38 (1934). Salomon, G., and Koningsberger, C., Rec. trav. chim., 69, 711 (1950).
Schlesinger, W., and Leeper, H. M., Science, 112, 51 (1950); IND.ENG.CHEM.,43, 398 (1951).
Direct hydrogenation of char provides
. , , an
insight into the mechanism of methane formation
. . .a potential method for producing high 8. t. u. 9-
Vol. 47, No. 4
(47) Schopfer, W. H., and Grob, E. C., Experientia, 8 , 140 (1952). (48) Staudinger, H., and Staudinger, Hj., J . prakt. Chem., 162, 148 (1943); Rubber Chem. and Technol., 17, 15 (1944). (49) Stewart, A. W., “Recent Advances in Organic Chemistry,” 5th ed., Vol. I, p. 292, Longmans, London, 1931. (50) Sturgis, B. M., Baum, A. A., and Trepagnier, J. H., IND.ENG. CHEM.,39, 64 (1947). (51) Weinstein, L. H., Robbins, W. R., and Perkins, H. F., Science, 120, 41 (1954). (52) Whitby, G. S., Kolloid-Z., 12, 147 (1913). (53) Whitby, G. S I “Plantation Problems of the Next Decade,” Weltervreden, Java, 1914; Ann. Botany (London), 31, 313 (1919); “Plantation Rubber and the Testing of Rubber,” pp. 5-7, Longmans, London, 1920. (54) Whitby, G. S., Dolid, J., and Yorston, F. H., J . Chem. SOC., 1926, p. 1448. (55) Whitby, G. S., and Greenberg, H., Biochem. J . , 35, 640 (1941); Rubber Chem. and Technol., 15,96 (1942). (56) Whitby, G. S., and Greenberg, H., IND.ENG.CHEM.,18, 1168 (1926). (57) Whitby, G. S., Gross, M. D., Miller, J. R., and Costanza, A. J., Chem. Eng. N e w , 29, 3952 (1951) ; J . Polymer Sci., in press, 1955; of. Willis, J. M., IND.ENG.CHEM.,41, 2276 (1949). (58) Wibaut, J. P., Rec. trav. chim., 62, 205 (1943). RECEIVED for review October 7,1954.
ACCEPTED January 5. 1955.
Kinetics of Carbon Gasification INTERACTION OF HYDROGEN WITH LOW TEMPERATURE CHAR AT 1500”TO 1700” F. C. W. ZIELKE AND EVERETT GORIN Pittsburgh Consolidation Coal Co., Library, P a .
N EXPERIMENTAL program has been in progress in these laboratories for some time on the kinetics of the gasification of low temperature char by hydrogen steam mixtures. Relatively large quantities of methane were obtained in these experiments ( 6 , 7 ) ,particularly at high pressures and high hydrogen-sfeam ratios. I n order to provide a better insight into the mechanism of methane formation, it was desirable to study the rate of gasification of char in pure hydrogen. This study is also of potential practical importance in connection with the production of high B.t.u. gas by the hydrogenation of coal or char. Some data have already been reported ( 7 ) on the hydrogenation of char a t 1600” F. These data have been extended to cover a wider pressure range and also two other temperatures. It has been long known that methane can be synthesized by the direct hydrogenation of carbon. The only extensive work that has been done previously in this field, however, is that of Dent and coworkers (2-4). These workers obtained integral rate data by heating various British coals and cokes in a batch bed to final temperatures of 800” to 900” C. under hydrogen pressures up to 50 atmospheres. When the final temperature was reached, i t was maintained constant while the hydrogenation was continued. It is only during this period that a comparison between their data and our data is possible.
EXPERIMENTAL TECHNIQUE
Procedure. The equipment and experimental procedure employed in this investigation were substantially the same as described previously for the char-steam kinetics work (6, 6). The inlet gas in the majority of the runs was pure hydrogen which was passed over a nickel catalyst for the removal of oxygen and through silica gel and magnesium perchlorate drying tubes before entering the reaotor. The feed char in all runs was the prepared 65- to 100-mesh Disco which was used in the previous char-steam kinetics studies. The char in each experiment was pretreated for one hour at the reaction temperature by fluidizing in purified nitrogen before the hydrogen was introduced. A fluidizing velocity of 0.44 foot per second was employed. Exit gas rates and gas samples were taken throughout the runs using pure hydrogen as the inlet gas while the temperature, pressure, and fluidizing velocity were held constant. The weight of feed char used in most runs was 0.4 pound. The reactor was the same ll/&ch diameter Uniloy reactor previously described (6). Besides the pure hydrogen runs, a series of experiments was conducted t o determine the effect of methane in suppressing the reaction rate b y adding methane to the inlet hydrogen. It was
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
April 1955
impossible to obtain accurate results by the standard experimental procedure in these runs since the variation in the methane content of the feed gas was in some cases almost as large as the incremental quantity of methane produced by reaction. The reaction was, therefore, followed by direct measurement of the amount of carbon gasification by analysis of the carbon content of the feed as compared with carbon content of the char residues. This procedure results only in an average gasification rate over the burnoff achieved that can be identified with a given inlet gas composition. The results are considered adequate, however, to define the inhibiting effect of methane. The gasifying time, the hydrogen partial pressure, and the temperature were held constant a t 100 minutes, 6 atmospheres, and 1600" F., respectively, while the partial pressure of methane was varied from 0 t o 0.9 atmosphere. A small 10-gram bed */, inch high was used in these experiments. They were conducted in a reactor liner of 1.22 square inch crom-sectional area to ensure complete recovery of the char residue. The hydrogen and methane were metered in separately. The mixing took place in the fixed gas line up stream of the reactor. Analytical Methods. Standard ultimate analyses were run on all bed residues. The exhaust gases were analyzed by the gravimetric ( I ) , infrared, and Tutweiler. methods to determine methane, carbon monoxide, carbon dioxide, and hydrogen sulfide. The infrared method gave the most precise values of the methane content of the exhaust gases, and this value was used in the calculations. A few tenths of 1% of carbon monoxide was detectable during the first few minutes of some of the runs. After this time, however, methane was the only carbon-bearing gas that was found. No ethane was detected by infrared analysis.
-
.
821
0
X
40R 50
I
\
I
I
1500
1600 1600 1600
30
N U
~
1
;
29.9 1.5 6.1 7.5 15.1 29.9
~
W
%
LL
+-
L
0
PER CENT CARBON GASIFIED
Figure 1. Hydrogenation rates as functions of process variables at 1500" and 1600" F. The only experimental quantity measured in the methane addition runs was the carbon burnoff achieved after 100 minutes running time. Therefore, a point rate cannot be calculated but only an average rate under the assumption that the gasification rate is independent of burnoff over the small range of burnoff experienced in these runs. The rate equation becomes =
RcH~W
or in the integrated form
EXPERIMENTAL RESULTS
Method of Processing Data. Abbreviated calculations showing the process used to obtain the gasification rates from the raw data--Le., the percentage of methane in the exhaust gas and the exhaust gas rates as functions of hydrogenation time-are given for a typical run in Table I. The rates are expressed as atoms of carbon gasified per minute per atom of carbon. The details of the calculational procedure are the same as reported previously in the char-Biteam studies (6).
Table I.
Tabulated Calculations for Run 111
Operating Conditions Temperature, F. 1600 Pressure. a t m . 29.9 Inlet gas (Hd, % 100 Initial bed wt , lb. atom C 0 02284 Inlet Hz rate, lb. mole/min. 5 60 X 10-8 Elapsed gasifying time, minutes Smoothed raw data Dry exit gas analysis CHI
0
50
140
260
WO
carbon present. The rate RCH,so defined is the gasification rate in terms of atoms of carbon gasified to produce methane per atom of carbon present per unit time. Graphical and Tabular Results. The integral rate data for the pure hydrogen runs are shown graphically as functions of carbon burnoff with pressure as the parameter a t 1500" and 1600" F. and at 1700" F. in Figures 1 and 2, respectively. The term carbon burnoff is used here synonymously with percentage of carbon gasified based on 100% carbon present at zero hydrogenation time. The equivalence of the integral dat;, given in Figures 1 and 2, with the true differential rates is shown by a comparison of the rates obtained in runs 109 and 114 where the initial bed weights
423 70,
HsS H:! (b.y diff.)
1.87 1.06 0.02 0.12 98.01 98.92
0.60 0.42 0.32 0.00 99.40 99:58 99 68
Y in4
59.20
59.20
Dry exit gas rate, l b . moles/min.
where F = ___ W 0 - is the fraction gasified and W is the weight of
59.20
:
59.07
58.77
Calcuiated quantities Integral gasification rate ( N ) , Ib. atoms C . pasif./min. X 104 110.7 62.75 35.52 24.81 18.81 Cumulative-C gasified, lb. atoms x 104 42.0 0 8 3 . 5 118.5 153.5 Instantaneous wt. C in bed ( W ) , Ib. atoms X 104 228.4 1 8 6 , 4 144.9 109.9 74.9 Specific gasif. Fate, , ( N / W ) , lb. atoms C gasif./rnin./lb. atom c x 104 48.5 83.7 24.5 22.6 25.1 C gasified, % 0 1 8 . 4 36.6 51.9 67.2 Methane equilibrium ratio, (P~dl/Pcna 1535. 2760. 4920. 7055. 9280.
The primary distinction between this work and the char-steam studies (6, 7 ) is that the integral rate data obtained as outlined above are equivalent to the differential rates and no extrapolation is required. This is due t o the small conversion of the hydrogen that is obtained and the fact, as shown later, that methane is not an inhibitor for the reaction.
*P x
PER CENT CARBON GASIFIED
Figure 2.
Hydrogenation rates as functions of process variables at 1700" F.
'IND U STRIAL AND ENG INEERING CHEMISTRY
822
Vol. 41, No. 4
DISCUSSION OF RESULTS
Empirical Correlation and Mechanism. The correlation of the reaction rate data is most conveniently carried out by first establishing the apparent order of the reaction. This may be done by plotting the reaction rate versus the pressure on a log-log plot. The 1600' and 1700' F. data are graphically represented in this fashion with carbon burnoff as parameter in Figures 3 and 4, respectively. A number of points in Figures 3 and 4 as well as in Figure 7 have identical ordinates and abscissas. The symbols for these points have been placed on their proper,ordinates but adjacent to each other to provide clarity in reading the graphs. The 1600" F. data a t 0 and 10% burnoff, the only two burnoff levels a t this temperature a t which more than two pressure levels were investigated, are well represented by a straight line-Le., the reaction order is independent of pressure level. The 1700' F. data cover a much wider range of carbon burnoffs but are available a t only three pressure levels. The data are fairly well represented by straight lines in this case also, but a closer inspection of the data shows that there is a tendency for the reaction order to decrease as the pressure is increased from 10 to 30 atmospheres as is illustrated.in Table V. mlo/lo and m 8 0 / ~ o are the average orders of the reaction in the pressure intervals from 10 to 20 and 20 to 30 atmospheres, respectively, calculated from the corresponding ratios of the reaction rates. The value used for the reaction rate a t 30 atmospheres was the average of the rates obtained in the two runs a t 30 atmospheres.
expressed in pound atoms of carbon were 0.02346 and 0.01327, respectively. The rates are substantially equivalent, which would not be so if extrapolation were required to obtain differential rates. The material balances for the various runs are given in Table 11. Some of the carbon balances deviate considerably from zero. This is probably a result of limitations of the analytical methods in analyzing for very small amounts of gas.
H/
20
Table 11. Materral Balances
y-)
Run No.
C
H
113 s-37 112 111
10.28 -1.49 -1.57 -5.66
-1.74 0.36 -2.24 -0.86
101 114 109
-3.68 8.59 -3.18
-2.52 -1.17 0.31
in
1.. .
-1.70
(100)
I
-1.03 . ..
10
I
1
1
20
25
30
40
Figure 4. Rate vs. pressure at 1700" F.
However, as was previously observed in the carbon steam kinetics work (7), the only correlation of the rate that fit even approximately over the whole range of data is the empirical equation R = DT" where T is the pressure and D and m are empirical constants that are functions of temperature and carbon burnoff but are independent of pressure. m is the apparent order of the reaction. The constants. D and m. were determined bv fitting the best straight lines to the data plotte; Table 111. Data for Methane Addition Runs in Figures 3 and 4 by the method of least squares (Temperature, 1600° F. : gasifying time, 100 minutes) where more than two points are available. m is Rate Atoms the rectangular slope of the straight lines and D pH* Inlet Gas pcr-14c,Initial Grams.Wt. ( W 0 ) c,Final GramsWt. (W) c Gasified, % 5.50 5.65 is the intercept with the ordinate. 6.1 0 7.299 6.898 6 . 0 0.30 7.273 6.951 4.43 4.53 The values of m and log D at 1600' and 6.0 0.58 7.252 7.068 2.54 2.54 6 . 0 0.90 7.239 7.231 0.36 0.36 1700" F. are plotted against carbon burnoff in Figures 5 and 6.
The average rates obtained in the methane addition runs are given in Table 111. The effect of particle size on the rate was determined by screening the bed residue from the run carried out a t 1600' F. and 7.5 atmospheres and determining the carbon burnoff reached in the different size fractions. The results are shown in Table IV.
Run No. 8-32 S-36A 8-38 8-35
I
15
PRESSURE (TO, ATM OSPHERES
*;$Jp;($
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1955
(Hydrogenation a t 1600' F. a n d 7.5 atmospheres Ha) Flanking Tyler Screen Sizes
Av. Particle Size, Microns
150-200 100-150 65-100
90 125 180
Table V.
I n the bonding arrangement pictured an edge group is generated rather than an
-CH-CH=
Effect of Particle Size on Gasification Rate
Table IV.
Carbons % Original Final
81.7 83.2 80.2
80.8 79.5 76.0
Ash, Original
12.5 12.0 14.9
70 Final
18.8 20.0 23.4
0
20
40
50
2.90 2.86
3.04 2.90
2.62 2.79
2.96 2.86
1.54 1.31
1.60 1.27
1.39 1.19
1.57 1.20
..
Aodication of r a t e equation Rzo/Rio (obsvd.) Rno/Rm (calcd.) Apparent order of reaction mm/1o
mso/zo
edge -CH=CHgroup. When this occurs resonance is assumed to regenerate the -CH=CHgroup. A more detailed consideration of such a 0.34 representation of the hydrogenation of the edge 0.43 0.40 of a graphite lattice yields the result that on the average an equal number of new active points of attack represented by the -CH=CHgrouping are generated for each one that is consumed-Le., the number of such groupings per unit of carbon is approximately constant. On the basis of this assumption, the assumption that reaction 4 is rapid compared with r&ctions 1, 2, and 3, and the assumption of the establishment of steady state concentration of the intermediate, B, the rate obtained is
Fraotion Gasified (F),7
1100" F. Rate Data, Apparent Order of Reaction, Application of Theoretical Rate Equation
Carbon gasified, %
823
(5) This correlation fits the data a t 1600" F. very well as shown in Figure 3. The fit at 1700" F. is not quite as good, however, because of the tendency mentioned for the reaction order to decrease with pressure. The fit is undoubtedly adequate, however, for engineering design purposes. The maximum percentage deviation, for example, between the experimental and calculated rates a t 1700" F. and 20% burnoff is only 5%. The correlation is, of course, strictly empirical and therefore cannot be used safely outside of the range of the variables investigated. It is difficult to reconcile the empirical correlation equation with one derived on the basis of a theoretical mechanism since such considerations inevitably lead to the development of a Langmuir-type equation. Such equations all have the single property in common that they predict a decrease in the order of the reaction with increasing pressure. As an example of a "reasonable" type of theoretical mechanism, one can imagine a mechanism based on the hydrogenation of the exposed edges of the graphite lattice as represented by the -CH=CHgroup pictured below: H
H
k ~ kz, , and ka are the rate constants for Equations 1, 2, and 3, respectively, and A represents the number of active groupings per unit of carbon. This expression would predict an apparent order of the reaction of 2 a t low pressures decreasing to a saturation value of unity a t high pressures. The correlation constants, as shown in Figure 6, indicate that usually the order of the reaction actually lies between the values of 1 and 2. There are a large number of other rate expressions that may be derived on the basis of alternate mechanisms that may be postulated for the reaction. They all show qualitatively similar behavior-Le., a maximum reaction order of 2 a t low pressures which decreases to a "saturation" value of either zero or one at high pressures. It is obvious that such a theoretical equation does not fit the data a t 1600" F. and 0 to 10% burnoff since here, as shown in Figure 3, the reaction order is independent of pressure. A possible explanation of this phenomenon is that a t the relatively low 'temperature and carbon hrnofl's involved, a conglomeration of different types of carbons is involved; these are hydrogenated by different mechanisms and therefore the independence of reaction' order is due to a fortuitous type of balancing and is without theoretical significance. At the 1700" F. temperature level and a t higher burnoffs, it would be expected that the carbon would become more uniform and approach graphite in its structure. Under these conditions, therefore, a mechanism represented by an equation, such as Equation 5, would be more applicable. This is obviously qualitatively true since, as was pointed out, the reaction order does indeed decrease with pressure as predicted,
0.2
0
0-0.2
a
H
H H
0
-I
-0.6
-1.0
0
Figure 5.
10
20
30
40
50
60
70
PER CENT CARBON GASIFIED Log of correlation constant, D , us. per cent
carbon gasified
INDUSTRIAL AND ENGINEERING CHEMISTRY
824
Vol. 41, No. 4
Temperature Coefficient. Hydrogenation runs were conducted a t all three temperatures a t only one common pressure level-30 atmospheres. The log of the reactioii rate :it this pressure level is plotted as a function of the recipro(d OF the abE 12 solute temperature with carbon burnoff as parameter i r i 1~’igule7 . Good straight lines are obtained that yield values for :t “pseudo” activation energy of the reaction as a function of oa,1k)oii burtioff 08 (Figure 8). The activation energy of the reaction iricrcwes fiith 70 10 PO 30 40 50 60 PER CENT CARBON GASIFIED burnoff and tends to level out toward an asymptotic. vitlut: tit high burnoffs. This is in line with the hypothesis IhaI the Figure 6. Correlation constant, m, vs. per cent carbon structure of the carbon residues approaches that of graplii Ie, gasified The term “pseudo-activation energy” is iisetl advisedly since the mechanism of the rcwvtioti ihi not known and it is, therefore, poss:ble t.li:i.I6 (,lie observed temperature coefficient i s the oorriposi t,e effect of several reactions occurring iri nequ(~ii(:e. Comparison with Previous Results. A tliscussion of the significance of the results rt?l:i,t,ive to the methane rates previously reporl,ecl ( 6 , 1’) in the hydrogen-steam system will be tlel’erred t o a future article in which a gerier.:ilized c:oi’relation will be given for the carbon gt~sifioa,I,ioii rates as a function of temper:tturn, p w w u r n , CARBON GASIFIED and hydrogen/steam ratio. A quantitative comparison of our Tesultjs wikh those previously obtained by Dent ant3 coworkers (2-4) is .in general not possible H i i i ( : c ? Dent’s work was not designed specifically to 01)tain a kinetic analysis of the hydrogeiiatioii reaction. Qualitatively the effect, of pressui’e .104 and temperature on the. rate as observed by T.“K. Dent, agrees with our work. A semiquantiLative Figure 7. Rate vs. reciprocal of absolute temperature comparison is possible only a t high burnoff levels. Only under these conditions did Dent, obtain simultaneously the constant temperature and low methane v o i i centration in the effluent gas that permitted identificatioii of their specific reaction rates with differential rates. For example, a t 50 atmospheres pressure and 70% burnof‘r, Dent obtained a gasification rate of 33 X 10-4 min-1 at 900” C. using a semicoke produced from a strongly caking British co:rl. This is considerably lower than the rate obtained from our char The extrapolated to the same conditions of 83 X 10-4 min.-’. discrepancy is perhaps attributable to the much lower ash content of their feed material. I6
Figure 8. Energy of activation 2)s. per cent carbon gasified at 29.9 atm. pressure The constants in Equation 5 may be consolidated and the equation rewritten as
Constants a and b were evaluated by fitting the equation to the two points a t 10 and 30 atmospheres. The ratio of the rates a t 10 and 20 atmospheres was then calculated and compared with the experimentally observed ratios. The comparison is given in Table V. Agreement is quite satisfactory-within
