Thermal Decomposition Rates of Carbonates in Oil Shale E. E. JUBKOLA, A. J. DENILAULER', H. B. JENSEN, W. I. BARNET2, AND W. I. R. MURPHY Petroleum and Oil Shale Experiment Station, U. S . Bureau of Mines, Laramie, Wyo.
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ETORTING oil shale involves heating it to a temperature over 750" F. to convert the organic matter to oil, gas, and coke. Depending upon the maximum temperature to which the shale is raised and the rate of heating, a portion of the mineral carbonates contained in the shale is decomposed. Calculations show that complete decomposition of the carbonates in a 30gallon-per-ton Green River oil shale would require a quantity of heat equivalent to that obtainable by burning approximately 10%of the organic matter in the shale (IS). Data on the extent and rate of this decomposition at various temperatures are essential in the design and operation of retorts for producing shale oil most economically and are the subject matter of this paper. LITERATURE
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Extensive studies of the decomposition of calcite have been made by various investigators. Although different results were obtained, the general conclusions show that calcite begins to decompose near 1650" F. when the mineral is in an atmosphere of carbon dioxide a t 760 mm. pressure (4,11-16). This dissociation of calcium carbonate into calcium oxide and carbon dioxide is a reversible endothermic reaction that Teaches equilibrium when the dissociation pressure of the carbonate equals the partial pressure of carbon dioxide in the atmosphere surrounding the sample. Kappel and Huttig ( 9 ) found that the reaction is of apparent first order during surface decomposition but changes to intermediate orders ( 2/a and l / g ) in the transition zone and finally to zero order in the central portion of the particle. Studies of dolomite decomposition show this mineral to be stable up to 1340" to 1420" F. (4-6, 11, IS) after which, as concluded by Schwab (13),the reaction to form calcium carbonate, magnesium oxide, and carbon dioxide occurred in a nearly explosive manner because the dolomite had remained in unstable .equilibrium above the dissociation temperature (1170 " F.) of magnesite. The calcium carbonate formed from this reaction seems to decompose similarly to calcite (11, 13). Berg (1) and 3chwab (13)showed that certain salts lower the dissociation temperature of dolomite, while Budnikov ( 8 ) found that salts increased the rate of carbon dioxide evolution by four and a half times a t 600" C. and two times at 700" C. Schwab concluded that addition of alkaline carbonates causes a premature decomposition of the dolomitic structure by catalyzing the decomposition of the dolomite into its two constituents. The dissociation then proceeds as if there were a physical mixture of the two carbonates, with no lowering of the dissociation temperatures of the individual components; this indicates that dolomite exists only in a metastable state above the dissociation temperature of magnesite and explains the explosive character of the primary dissociation. Esin, Gel'd, and Pope1 (6) noted that sodium and potassium chlorides, sodium nitrate, sodium fluoride, sodium oxalate, and to a lesser degree sodium carbonate, when placed in contact with dolomite increased the intensity of carbon dioxide evolution a t a given temperature and concluded that it was due to the salts aiding redistribution of the ions so that the beginning of carbon dioxide evolution was shifted to lower temperatures. Other factors influencing the rates of decomposition of car1 Present
address, Dow Chemical Co., Denver, Colo. Brea, Calif.
* Present address, Union Oil Co.,
bonates are particle size, molecular structure, and composition of the surrounding atmosphere (9, 10, 12). The rate of decomposition increases with smaller particle size because of the shorter time needed for heat to penetrate to the reaction zone and the greater ease with which the carbon dioxide diffuses through the particle and away from the reaction zone (10). A similar effect was attributed to the molecular structure-that is, crystalline varieties of carbonates that have more closely packed molecules decompose somewhat more slowly than amorphous varieties (11, 12). Bischoff's work with gases ( 2 ) showed that heating of magnesite a t 500" C. and calcium carbonate at 725" C. in the presence of ammonia, water vapor, hydrogen, and nitric oxide caused nearly complete decomposition of carbonates as compared to only partial decomposition in the presence of helium, nitrogen, carbon monoxide, and oxygen, and under vacuum. With similar conditions, practically no decomposition occurred in an atmosphere of carbon dioxide. Huttig (8) concluded that, where no chemical or catalytic effect occurred, the carbon dioxide evolution rate increased inversely with molecular weight of the ambient gas. In this category he included helium, argon, nitrogen, oil, and carbon monoxide while he listed hydrogen, ammonia, water vapor, and nitric oxide as having a chemical or catalytic effect. In summary, the literature shows that calcite dissociation begins near 1650" F. and dolomite near 1420 F. when heated in an atmosphere containing carbon dioxide a t a partial pressure of 760 mm. The presence of certain salts or gases lowers these dissociation temperatures. The rate of decomposition also depends on factors such as particle size and crystal form. Because of the effect of all the8e variables, many of which may be present in a retorting system, it seemed advisable to conduct a study of the rate of decomposition of carbonates in oil shale rather than to depend on published data for pure minerals. O
APPARATUS
Oil shale, in contrast to pure mineral carbonates, contains volatile organic matter and water, the presence of which eliminates the usual method for determining the degree of decomposition by measuring the volume of carbon dioxide evolved or the loss in weight of the sample. Thus, it was necessary to design special apparatus and to use analytical methods suitable for oil shale. The apparatus used consisted of a furnace, reactor, sample holder, gas supply, drying tube, gas meter, temperature controller, voltage transformer, and high speed temperature recorder. Figure 1 shows the reactor and furnace assembly. The furnace consisted of an alundum core, 4 l / ~inches in inside diameter by 111/, inches in length wound with Chrome1 A heater wire having an output of 800 watts. This core was placed in an 8inch Transite pipe and the annular space filled with magnesia insulation. The reactor tube was fabricated from stainless steel and contained an integral gas preheater made by using a section of 3-inch standard pipe in which was placed a longer section of 2-inch standard pipe. The annulus was sled with stainless steel turnings and, when the ends were closed and an inlet and outlet provided, formed the gas preheater. The inner tube, partly filled with stainless steel helices, was the reactor. A sample holder was fabricated from 1-inch stainless steel tubing provided with a gas tube having a female joint. The preheated gas entered this tube and was conveyed to the bottom of the sample holder. The gas then passed through a perforated metal 2'111
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2712 CATHERMOCOUPLE TO RECORDEfi
7 YTHERMOWELL
I ' S T D PIPE X 10~5/S'LONO CA THERMOCOUPLE TO CONTROLLER
2"STD. X 13.3/4"LONG STAINLESS STEEL PIPE STAINLESS STEEL LATHE TURNINGS 8"TRANSITE PIPE X l2.3/4*LG.
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tory. Experiments were then carried out with a sample of oil shale from the Bnvil Poirits mine a t Rifle, Colo. This oil shale had a Fischer assay of 28 galloas of oil per ton and an organic content of 15%, the remaining 85% being mineral matter. The mineral matter was composed of 53% carbonates, the remainder being ewentially clays, quartz, and feldspars. The approximate mineral composition, shown in Table I, was calculat.ed from the chemical analysis by assuming the presence of only those minerals that were identified by x-ray analysis. Nearly 64 mole % of the carbon dioxide in the mineral matter is present in dolomite and the remaining 36 mole % in calcite. The oil shale was ground to 98% -65 mesh and contained less than 27, -270-mesh material. Carbonate decomposition rates were determined for the oil shale a t 50" intervals from 1050" to 1600" F. in both an atmosphere of carbon dioxide and of nitrogen. DISCUSSION O F RESULTS
SECTION A.A
800-WATT HEATING ELEMEhT NO.15 CHROMEL*i' WIRE IRESISTANCE -020 OHMS/FTl STAINLES5 STEEL HELICES
Figure 1.
Data for the decomposition of the carbonates in oil shale are plotted in Figures 2A and 2R. Comparison of these curves shows that, for temperatures from 1050' to 1200' F., the early part of the decomposition progresses at a faster rate in the carbon dioxide than in the nitrogen atmosphere. However, cumulative decomposition a t 1150' and 1200" F. reaches the Same value aft'er a period of time. Increased rates of deconiposit'ion of carbonates with incremed
Reactor and Furnace Assembly for Carbonate Decomposition Studies
TABLEI. C.4LCUL.kTED disk, percolated through the sample, and passed out of the system. A combination reactor cap and thermowell protector tube, to which the sample holder was pinned, was used for placing the sample holder into the reactor.
llineral Dolomite Calcite
Plagioclase Illite
EXPERIMENTAL WORK
Silica Analcite
COMPOSITION OIL SHALE
LfINERAL
OF
COLORADO
Total Mineral Matter, yo 33 20 12 11 10 7 4 2 1 100
The experimental procedure consisted of heating the reactor to Orthoclase Iron the desired temperature, beginning the flow of sweep gas (about Pyrite 650 to 700 cc. per minute, which was equivalent to slightly more Total than one complete sweep of the reactor volume each minute), and when temperature equilibrium was reached, inserting the sample holder containing 10 grams of IO0 sample into the reactor. The temperature was held constant during the run by means of an automatic temperature controller and a manually operated voltage transformer. A Chromel-Alumel 5 80 thermocouple, inserted into the center of the sample and connected to a high speed, electronic, W continuous-line recorder, gave a complete tempera5, ture history of the sample during the test. The 0runs were terminated by withdrawing the sample p holder and partly quenching it in water. The 40 percentage of carbonate decomposition was calculated from determination of carbon dioxide rep U maining in the sample. P 20 The initial work was conducted t o test the appa0 ratus, 66 runs being made with precipitated cal6s-270 MESH cium carbonate or v-ith calcite crystals ground to -60 mesh. The data thus obtained showed that 40 30 I20 160 200 240 280 320 00 the decomposition rates followed those predicted TIME-MINUTES in the literature and thereby demonstrated that Figure 2A. Thermal Decomposition of Carbonates i n Oil ShaIethe apparatus and technique used were satisfacCarbon Dioxide Atmosphere, 1050-1600" F.
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December 1953
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TABLE11. REACTION-RATE CONSTANTS FOR DECOMPOSITION OF
CARBONATES I N OIL SHALE
Reaction-Rate Constant. k. in Reoimocal Minutes Temperature F.
OR.
(2')
1510 1560 1610 1660 1710 1760 1810 1860 1900 1960 2010 2060
1
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lo' 66.2 64.1 62.1 60.2 58.5 56.8 55.2 53.8 52.6 51.0 49.8 48.5
MgCOa NZatm. COZatm.
0.000380 0.000760 0.00125 0.00299 0.00940 0.0400
......
...... ......
...... ...... .. t . .
0.00200 0.00511 0.00769 0.00907 0.0158 0.0297 0.0335 0.0420 0.0629 0.0814
....... .......
CaCOa Nz atm. COz atm.
..... .....
..... .....
...... ...... ......
...... ...... . . . . . 0.000633 0.00416 0.00104 0.00235 0.0118 0.0353 0.0732 0,1292
.....
0.00282 0.00760 0.0210 0.131 0.196
2
carbon dioxide content of the atmosphere appears anomalous. At 1250" F. and higher, decomposition of about the first 30 mole % of the carbonates appears to occur a t essentially the same rate in both atmospheres, but further decomposition is more rapid in nitrogen than in carbon dioxide. These results indicate that the dolomite first decomposes into magnesium oxide, carbon dioxide, and calcium carbonate. The decomposition of magnesium carbonate is essentially a nonreversible reaction-that is, the partial pressure of carbon dioxide required to reverse the reaction is so much higher than that existing in the present experimental work that varying it from about 0 to 585 mm. of mercury has no appreciable effect on the magnesium carbonate decomposition rate ( 7 ) . The calcium carbonate from the dolomite and the calcite decomposes a t a somewhat slower rate, thus causing the considerable change in slope at about 30 mole % carbonate decomposition, particularly for the 1200' to 1300" F. turves. The reaction rate is increased by decreasing the partial pressures of carbon dioxide, as indicated by changing from a carbon dioxide to a nitrogen atmosphere. Samples of both oil shale and dolomite that had been heated until the carbon dioxide evolved equaled the mole % of carbon dioxide present in the magnesium carbonate were submitted to x-ray analysis. The diffraction patterns obtained showed complete removal of dolomite and an apparent increase in calcite. These results agree with those of Ralston et al. ( 1 2 ) , in that the magnesium carbonate breaks down first and at a much more rapid rate than the calcium carbonate. The carbonate decomposition data for oil shale in an atmosphere of carbon dioxide are plotted as a first-order reaction in Figure 3. Two rates of decomposition are apparent as a result of
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Figure 3. First-Order Plot of Carbonate Decomposition in Oil Shale
the relatively large differences in the dissociation temperatures of dolomite and calcite. Therefore, t o determine the amount of dissociation occurring at a given temperature in a certain length of time, the individual reaction rates for the magnesium carbonate and calcium carbonate portions of dolomite must be utilized. The reaction-rate constants, k , calculated from these curves are shown in Table 11. The dissociation rate of magnesium carbonate wasf3 calculated from the rate a t which 0 to 25% of the total carbon dioxide in the oil shale was evolved, while the rate of decomposition of calcium carbonate was calculated from the rate of evolution beyond 32% of the carbon dioxide in the sample. The rate of decomposition between 25 and 32?& being near the transition zone, was not included with either group of data. The decomposition-rate constants obtained in this manner are plotted in Figure 4 against reciprocal of absolute temperature. -As the slopes of t h e lines representingthe rate of calcium carbonate decomposition are parallel, the energies of activation are the same whether 'the reaction takes place in a nitrogen or carbon dioxide atmosphere. However, the nitrogen atmosphere nearly quadrupled the rate of carbon dioxide evolution. On the other hand, more energy of activation is required to decompose the dolomitic magnesium carbonate in a nitrogen atmosphere than in a carbon dioxide atmosphere. I n either case the energy is less than that required to decompose the calcium carbonate. The rates of calcium carbonate decomposition are plotted in Figure 5 against the reciprocal of absolute temperature. Literature data for limestone and calcite along with data obtained in this study for calcite and precipitated calcium carbonate are included in this plot. These data indicate Arrhenius activation energies, E, of 140,900 and 89,200 cal. per gramT I M E - MINUTES mole, respectively, for decomposition of calcite Figure 2B. Thermal Decomposition of Carbonates in Oil Shaleas the mineral and when present in oil shale. . Nitrogen Atmosphere, 1050-1600° F.
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INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY RECIPROCAL OF ABSOLUTE T E M P E R A T U R E X lO'.*R
Vol. 45, No. 12
RECIPROCAL OF ABSOLUTE T E M P E R A T U R E X 10.'
*R.
Cor ATMOSPHERE
I
I100 T E M P E R A T U R E , .F.
Figure 4. Decomposition Rates of Carbonates in Oil Shale CONCLUSIONS
Dolomite in oil shale begins to decompose somewhat below 1050" F. while the calcite begins to dissociate in the range 1150" to 1200 F. Both dissociation temperatures appear to be nearly 450" F. lower than that of the corresponding carbonate in a relatively pure state. The decomposition of calcium carbonate in oil shale attains a reaction rate about 200 F. below the temperatures corresponding to the same reaction rate in pure calcium carbonate. X-ray analysis showed the dolomitic magnesiumcarbonate in oil shale to decompose first after which an abrupt decrease in the rate of decomposition is apparent. At temperatures below about 1200' F. decomposition of the remaining carbonates is extremely
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IS00 1700 I600
1600
TEMPERATURE,
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Figure 5. Decomposition Rates of Calcium Carbonates
done under a cooperative agreement between the University of Wyoming and the Department of the Interior, Bureau of Mines.
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The atmosphere surrounding the oil-shale particles has an anomalous effect on the decomposition of the carbonates. A nitrogen atmosphere favors the decomposition of calcium carbonate by reducing the partial pressure of carbon dioxide which is an end product of a reversible reaction. The opposite effect is observed in the decomposition of the dolomitic magnesium carbonate. ACKNOWLEDGMENT
This project was part of the synthetic fuels program of the Bureau of Mines and was performed a t the Petroleum and Oil Shale Experiment Station, Laramie, Wyo., under the general supervision of H. P. Rue and H. M. Thorne. Special thanks are due various members of the personnel of the station for their valuable assistance in carrying out this project The work was
LITERATURE CITED
(1) Berg, L. C., Compt. rend. acad. sei. U.R.S.S., 38, 24-7 (1943). (2) Bischoff, F,, Radex-Rundschau, 1949, 8-13. (3) Budnikov, P. P., and Bobrovnik, D. P., J . A p p l . Chem. (U.S.S.R.), 11, 1151-4 (1938). (4) Conley, J. E., Am. Inst. Mining M e t . Engrs., Tech. Pub. 1037 (1939). (5) Cuthbert, F. L., and R o d a n d , R. ii., Am. Mineral., 32, 111-16 (1947). (6) Esin, 0. A., Gel'd, P. V., and Popel, 9 . I., J . A p p l . Chem. (U.S.S.R.), 22,354-60 (1949). ( 7 ) Haul, R. A. W., and Heystek, H., Am. Mineral., 37, Nos. 3 and 4, 166-79 (1952). (8) Huttig, G. F., and Heina, H., Z . anorg. cham., 255,223-37 (1948). (9) Kappel, H., and Huttig, G. F., Kolloid-Z., 91, 117-34 (1940). (10) Maskill, Wm., and Turner, W. E. S., .J. SOC.Glass Technol., 16, 80-93 (1932). (11) Potapenko, S. V., J . Applied Chem. (C'.S.S.R.), 5 , 693-704 (1932). (12) Ralston, 0. C., Pike, R. D., and Duschak, L. H., Bur. of M i n e s Bull. 236 (1925). (13) Sohwab, Yvan, Compt. rend., 224, 47-9 (1947). (14) Schwab, Yvan, Rev.mate'riaux construction trav. publ., ed. C. No. 411,409-20 (1949). (15) Southard, J. C., and Royster, P. H., J . Phys. Chem., 40, 435-8 (1936). (16) U.S. Bur. M i n e s , Rept. 1nl;est. 4771,60 (1951). RECEIVED for review May 4, 1953. ACCEPTED July 13, 1953. Presented before the Diviaion of Industrial and Engineering Chemistry a t SOCIETY, Los Angeles, Calif. the 123rd Meeting of the AMERICAN CHEUICAL
