552
Ind Eng Chem. Process Des Dev 1986, 2 5 , 552-557
Catalytic Effects of Transition Metals on Oil Shale Pyrolysis Noreddlne Torels, Xenophon E. Veryklos, and Ellhu Grossmann Department of Chemical Engineering, Drexel University, Philadelphia, Pennsylvania 19 104
The decomposition of kerogen of Green River formation oil shale, impregnated with transition metals, was investigated following nonisothermal thermogravimetric techniques, in the presence of hydrogen at atmospheric pressure. Experimental data were best fit by a first-order kinetic model with respect to kerogen. The effects of ambient atmosphere and heating rate were also investigated. In the absence of metals, inert and reducing atmospheres were found not to affect kinetic parameters. The frequency factor was found to increase with heating rate at a constant activation energy. The presence of metals in the oil shale matrix and hydrogen in the retorting atmosphere did not alter the kinetic order of kerogen decomposition, but it lowered apparent activation energies and increased the rate constants. These results are discussed in terms of dissociative hydrogen adsorption on the metal crystallites and hydrogen spill over to the organic matter, resulting in hydrogenation andlor cracking reactions.
1. Introduction Oil shale is the name given to finely textured mineral deposits containing high molecular weight organic material. A small fraction of the organic material is soluble in organic solvents, while the major fraction is a waxy substance of high molecular weight, commonly referred to as kerogen. The typical kerogen molecule is a polymer with a molecular weight well above 3000, with a structure which is highly naphthenic with closely associated aromatic, nitrogen, and sulfur heterocyclic ring systems (Allred, 1966). This insoluble organic fraction is the basic raw material for production of oil, which after appropriate treatment is suitable for use as refinery feedstock. Thermal decomposition of kerogen to yield oil, gases, and char occurs at temperatures above about 623 K. The kinetics of the decomposition of kerogen have been the subject of numerous investigations, employing both isothermal and nonisothermal techniques. The complex chemical structure of kerogen has led to different kinetic postulates, ranging from simple first-order reaction schemes (Arnold, 1976; Haddadin and Tawarah, 1980;Shih and Sohn, 1980) and a second-order reaction (Mickelson and Rostani-Abadi, 1980) to complex mechanisms consisting of a series of first- and second-order and autocatalytic reaction steps (Wen and Yen, 1977). Hubbard and Robinson (1950) correlated their kinetic data in terms of two consecutive first-order reactions: kerogen bitumen oil gas + residue. Braun and Rothman (1975) interpreted these data under the assumption that both the decomposition of kerogen and the formation of carbon residue follow the same reaction sequence and introduced a thermal induction period. Weitkamp and Gutberlet (1970) interpreted their kinetic data by a mechanism based on a diffusion-limited first-order reaction complicated by the possibility of bond-breaking steps. In some cases, it has also been postulated that the pyrolytic bitumen has an autocatalytic effect on the decomposition of kerogen (Allred, 1966). In a detailed study of the evolution of different gases during kerogen pyrolysis (Campbell et al., 1980a, b), hydrogen was shown to be the major noncondensible gas, its evolution postulated to occur in three steps. The first one was associated with oil release during bitumen to oil pyrolysis, while subsequent releases were associated with
-
* To whom
-
+
correspondence should be addressed.
secondary pyrolysis reactions of residual carboneous residues a t temperatures above 873 K. Methane evolution was almost the same as that of hydrogen, while evolution of C2 and C, hydrocarbons did not exhibit any secondary releases. In the development of ex situ oil shale utilization technologies, most of the work has concentrated on sequential efforts to retort the shale and then to collect and treat the oil obtained (Perry, 1981). Some attempts have been made to combine pyrolysis and hydrotreating in the same unit. Thus, according to the patent literature, retorting and hydrocracking, using activated spent shale, have produced high yields of low boiling point liquid hydrocarbons or of a gas suitable for subsequent production of methane or SNG (Wysluozil, 1976). Hydroretorting, consisting of heating the oil shale at temperatures between 723 and 948 K in the presence of hydrogen, has also been used to produce SNG or a liquid-distillatehydrocarbon and a small proportion of low molecular weight paraffinic hydrocarbon gases (Weil et al., 1977). Retorting and oil cracking have also been combined within the same unit in two separate sections (Linden et al., 1975). The effects of hydrogen in the retorting atmosphere have been studied by many investigators. Larson and Wen (1981),conducting oil shale pyrolysis in an atmosphere of hydrogen at pressures up to 4.8 MPa, have shown that the hydrogen-to-carbon ratio in the residue is decreased due to preferential cracking of hydrogen-rich fragments of the total organic matter. However, the same decline in the H/C ratio of oil shale organic material soluble in toluene suggests that hydrogenation reactions of the liquid product might occur a t temperatures above 623 K. Schlinger and Jesse (1967) have demonstrated that use of hydrogen at pressures between 68 and 135 atm results in shale oil recovery far in excess of the modified Fisher Assay. Similar beneficial effects of hydroretorting, either in shale oil yields or product quality, have been reported by other investigators (Huntington, 1966; Tarman et al., 1977; Weil et al., 1976). Inclusion of other gases in the retorting atmosphere has also proven to be useful in enhancing both the quality and quantity of oil produced by oil shale pyrolysis. Thus, Cummins and Robinson (1975) demonstrated the effectiveness of mixtures of CO and steam in hydrogenating oil shale residues, while Leaman et al. (1979) used supercritical steam to produce a petroleum-like pyrolyzate from oil shale. Supercritical steam has also been found (Weil et
0196-4305/86/1125-0552$01.50/00 1986 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 2, 1986
Table I. Impregnating Solutions metal Ni co Mo Ir Rh Ru Pd
Pt Ni-Mo CO-MO
salt solution Ni (N03)2-6H20(1) Co (N03)2.6H20(2) ammonium molybdate (3) IrC13.3H20 Rh(N03)2,2HzO Ru(NO).(N@)3 tetraaminepalladium(I1) nitrate chloroplatinic acid mixtures of (1)and (3) mixtures of (2) and (3)
al., 1976) to substantially increase hydrogen production and to decrease CO content of the off-gas stream, probably due to the shift reaction. The purpose of this study was to determine if the presence of transition metals and hydrogen in the retort would influence the decomposition of kerogen. Transition metals have been used in coal liquefaction and gasification processes as catalysts for hydrodesulfurization, deoxygenation, and dehydrogenation as well as for olefin saturation to equivalent paraffins and breaking of long-chain molecules. Because of many similarities between coal liquefaction and oil shale pyrolysis, some catalytic effects of metals in the latter might be expected. 2. Experimental Section The samples used in the kinetic experiments were prepared by crushing the oil shale and then grinding in a ball mill until the desired particle size was obtained. In order to eliminate the influence of natural bitumen on the decomposition of kerogen and to simplify the kinetic analysis, the samples were extracted at room temperature, first with chloroform and then with a mixture of benzene, methanol, and acetone a t molar ratios of 5:l:l. A Soxhlet extractor was used for this purpose. The extracted samples were then dried at 378 K to evaporate any adsorbed solvents and free moisture originally present in the sample. Final drying was carried out prior to each kinetic run a t 423 K in flowing nitrogen. The nonisothermal Thermal Gravimetric Analysis (TGA) technique was used to study the kinetics of thermal degradation of kerogen in oil shale at atmospheric pressure. A Du Pont 950 module equipped with a temperature programmer and controller and the 900 D.T.A. recorder were used. Our choice of the nonisothermal kinetic method was motivated by its advantages over the isothermal one: (1)elimination of the heating-up period is required to reach the isothermal temperature; (2) one experiment can be used to evaluate all kinetic parameters; and (3) the entire decomposition process is used in the kinetic analysis. Metals were introduced into the oil shale particles, after extraction of bitumen, by the incipient wetness technique. The metal salt solutions which were used for this purpose are detailed in Table I. In each case, the oil shale particles were mixed with an aqueous solution of the metal salt of an appropriate amount and concentration so as to obtain the desired metal content on the oil shale. Since the solutions did not wet the oil shale particles, wetting was achieved by adding a few drops of a surfactant to the solution. The slurry was maintained at a temperature of 333 K with stirring until the solution was soaked into the particles. The impregnated samples were then dried in an oven overnight at 373 K and then reduced in a stream of hydrogen a t 423 K for a period of 4 h. The possibility of kerogen decomposition during reduction of the metal salts to the corresponding metals was investigated by using identical reduction conditions in a TGA apparatus. The weight loss which was observed over
553
a period of 4 h was approximately equal to the weight loss expected from the decomposition of the metal salts to the corresponding metals, indicating that no appreciable decomposition of kerogen occurred during this treatment. On the other hand, kerogen decomposition was observed when the metal salts were reduced at 473 K for the same period of time. 3. Theory In the present study, kinetic data are analyzed by power-law formulations of the rate of decomposition of kerogen. Complicated kinetic expressions are avoided primarily because our main purpose is to investigate any catalytic influence of transition metals on the effective kinetic parameters. Thus, in the model proposed, it is assumed that the mechanism of the decomposition of kerogen and the concentration dependence of the rate of volatilization are the same over the entire composition range. Both the integral method and the differential method for nonisothermal kinetics are used to analyze the data. Considering a global reaction scheme,
oil shale
-
oil + gas
+ residue
(1)
the rate of decomposition of kerogen is given by dx -- kx" dt where x =
w -w, wo - Wf
(3)
W , is the initial weight of the sample, W is the sample weight at any time, and W, is the final sample weight, at the end of reaction, and n is the order of the reaction. The reaction rate constant, lz, is assumed to follow the Arrhenius temperature dependence. If the heating rate is constant, dT = a (4) dt Equations 2-4 can be combined to give --dx = A e - E / R T X n dT a
(5)
Equation 5 can be rearranged to a linear form:
A plot of In (-dx/dT/xn) vs. 1/T would result in a straight line for the correct value of n. From the slope and the intercept, the values of the apparent activation energy and the frequency factor can be obtained. Values of -dx/dT at various temperatures are obtained from the TGA curves. To utilize the integral method of analysis, eq 5 can be integrated by means of a series expansion (Arnold, 1976; Johnson and Tolberg, 1982) to obtain
or for n = 1
Equations 7 and 8 can be solved for the value of x predicted by the model, x,, as a function of temperature and can be compared with the experimental values of x over
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Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 2, 1986
Table 11. Oil Shale Elemental Analysis element wt % element 20.41 N total C" H S 2.24 0 7.19 organic C b
2 '
1
n: 2 4-n:22/3
-0-
wt%
1.83 0.24 17.29
-
0 -
\
the entire course of the reaction. Thus, after some algebraic manipulations,
v
:
0.5
1
I
Cx I
Using an oxidizing atmosphere at 1373 K. *Using a filament (hot wire) at 873 K.
-D-
--24
t
-6
-8 1.20
1.32
or
1.56
1.4A
lOOo/T
1.68
Figure 1. Differential analysis for reaction orders of 0, 1/2, 2.
where
c = In
1.80
, K-l 2/3,
and
[ %( e ) ] 1 - 2RT
Equations 9 or 10 can be used to generate weight vs. temperature curves which can be compared with the experimental curves. 4. Results and Discussion (a) Chemical Characteristics of Oil Shale. Green River formation oil shale from the Uinta Basin was used in this study. It was characterized by Fisher Assay, X-ray diffraction, and elemental analysis. The Fisher Assay analysis indicated an oil shale which yields 32 gal of oil per ton, or 12.4% by weight. The X-ray diffraction analysis showed that the mineral constituents exist as carbonates (calcium-magnesium, 45%, calcium, 20%) and silicates, 20%. No pyrite was detected, in contrast to some other oil shales. The elemental analysis, which is shown on Table 11, indicates that the main elements are carbon, hydrogen, oxygen, nitrogen, and sulfur. Eighty percent of the total carbon was found to be organic. As was expected, the nitrogen content is high, approximately 29'0, while the sulfur content is low, around 0.2%. (b) Elimination of Intraparticle and Interparticle Diffusional Limitations. To obtain meaningful kinetic data on the decomposition of kerogen, any effects of mass and heat diffusional limitations must be experimentally eliminated. Since oil shale has its organic matter distributed within its inorganic mineral matrix, the kinetics of pyrolysis might be affected by heat- and possibly mass-transfer limitations. Preliminary experiments were run to determine the maximum oil shale particle size below which intraparticle diffusional limitations would be negligible. A definite inverse relation between decomposition rates and mean particle size of the sample was observed. The observed lag in weight change vs. temperature and its increase as the particle size increased may be attributed to the poor thermal conductivity of the mineral matrix. Thus, the rate of release of volatile products is controlled by the overall temperature gradients inside the particles when larger particles are employed. Weight loss vs. temperature curves were found to be nearly identical when particles of an average diameter less than 0.5 mm were employed in the experiments. Thus, in all kinetic experiments, the average particle diameter employed was 0.36 mm to ensure the absence of intraparticle diffusional limitations. Another transport process which might be expected to affect the observed kinetic parameters is interparticle transport of heat. To investigate this parameter, a number
1
t -6
I
12 0
132
1.41,
1.56
1.68
1.80
1000/1 , K-'
Figure 2. Differential analysis for reaction order of unity. (a) N,, 25 K/min. (b) H2, 10 K/min. (c) N P , 10 K/min.
of experiments were conducted in which the initial sample weight was varied between 20 and 100 mg, while all other parameters, including particle size, were kept constant. It was determined that when a sample mass larger than approximately 60 mg was used, interparticle transport resistances decreased the rate of decomposition of kerogen. This is probably due to the fact that temperature gradients develop within the sample mass and the temperature at the center of the sample is lower than that at the outer layers of the sample. In all subsequent experiments, a sample mass of 50 mg was used. (c) Estimation of Kinetic Parameters. In order to quantify the catalytic effects of transition metals on the rate of volatilization of kerogen, kinetic parameters must be established in the absence of such metals. The thermogravimetric technique is not sufficient for the purpose of establishing kinetic models since reactions which do not lead to volatile products are not detected. Nevertheless, the purpose of this study was not to develop a kinetic model for the decomposition of kerogen but to determine the effech of transition metals and hydrogen on the overall kinetic parameters of kerogen decomposition/volatilization. The TGA technique employed is suitable for this purpose. Rates of kerogen decomposition a t various temperatures were computed by drawing tangents to the TGA curves at the points of interest and computing the slope of those tangents. The TGA curves were sufficiently smooth to permit accurate drawing of the tangent lines. To obtain the order of the decomposition of kerogen, based on weight loss, various values of n between 0 and 2 were substituted into eq 6 and the resulting expressions were tested against the experimental data. Figure 1gives the results of values of n of 0, 1/2, 2/3, and 2. Similar plots, obtained under different experimental conditions, for the value of n equal to 1are shown on Figure 2. It is apparent from these graphs that a first-order reaction best describes
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 2, 1986 555
,
100
2o
--d25'K/mir!
I
,
I
1 6 ' K / min
3
I Model
::. z
^*
II +N,,
4'~/mln
I
I
, 533
573
1
613
653
573
1
593
733
1
613
653
693
73 3
773
773
Temperature , ' K
Figure 3. Comparison of experimental and predicted values by first-order reaction model.
the experimental data of the decomposition of kerogen in the range of temperatures studied, 473-773 K. Similar results were obtained for all other experimental conditions used in this study. The choice of the kinetic model and the order of the reaction can be tested by using the integral method of analysis, according to eq 10. Plots of xm, according to eq 10, are shown along with experimental values of x on Figure 3. An excellent agreement is observed between the experimental and the predicted kinetic behavior, indicating that the proposed model adequately describes the global kinetics of the volatilization of kerogen under all heating rates and pyrolysis atmospheres employed in this study. Values of activation energies calculated according to the first-order kinetic model are in the order of 30 kcal/gmol. These values are smaller than those expected from cracking-type reactions. Nevertheless, it has been shown (Anthony and Howard, 1976) that a set of overlapping independent parallel first-order reactions can be approximated by a single-order expression having both a lower activation energy and lower frequency factor than any of the reactions in the set. I t is possible that the same principle is responsible for the observation of low activation energies and frequency factors in the kinetics of the decomposition of kerogen. (a) Effects of Heating Rate and Atmosphere of Pyrolysis on Kinetic Parameters. To investigate the effects of the heating rate on the observed kinetic parameters, a number of experiments were conducted in which the heating rate was varied from 4 to 25 K/min. An apparent lag, observed on the TGA curves, which increased with the increasing heating rate indicated that during the nonisothermal degradation of kerogen, not only the temperature but also the amount of heat received by the sample per unit time affects the kinetics of the decomposition reactions. On Figure 4,the rate of volatilization is shown to increase with increasing temperature, go through a maximum and decrease at higher temperatures. Furthermore, the maximum rate of volatilization increases with an increasing heating rate. The temperature at which the rate is maximum, T,, is either independent or increased lightly with the heating rate as shown on Table 111. Frequency factors are shown on Figure 5 to be significantly affected by the heating rate, increasing as the heating rate increases. On the other hand, the activation energy, also shown on Figure 5 , was found to be independent of the heating rate, indicating that the mechanism
Temperature
I
'K
Figure 4. Effects of heating rate on rate of volatilization. 6
5
1
1 '
T
,
I
4 3
" %
W z 3 0
a
I
11 I
hEATlNG
RATE ,
'K/mln
Figure 5. Effects of heating rate on frequency factor and activation energy. Table 111. Effects of Heating R a t e on Rate of Volatilization max rate 70. min-' heating rate, K/min temp of max rate, T,, K 4 10 16 25
2.6 6.2
9.8 18.2
713 721 721 729
by which the decomposition of kerogen proceeds is not affected by the heating rate. The observed increase of the frequency factor with the increasing heating rate a t a constant activation energy might be the result of coking reactions which occur a t slower heating rates. Due to longer residence times at lower heating rates, decomposition reactions occur at lower temperatures, producing oil molecules which do not volatilize but continue to react. Thus, the thermogravimetric technique itself is contributing to this phenomenon since only volatile products contribute to the observed rate. Any reactions which lead to nonvolatile products at a particular temperature are not detected. To investigate the influence of the atmosphere under which the pyrolysis is taking place, on measurable kinetic parameters, a series of experiments was conducted under atmospheres of nitrogen, hydrogen, and equimolar mixtures of nitrogen and hydrogen. In all cases, the first-order kinetic model fit the data well. Activation energies and frequency factors were found not to be affected by the presence of hydrogen in the retorting atmosphere. This observation indicates that there is no participation of
556
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 2, 1986
Table - IV. Parameters of Kerogen Decomposition in the Presence of Metals and Hydrogen metal metal loading '70 E, kcal/gmol A , min-' 102k6,,, min-' limiting value % 31.1 5.4 x 108 4.3 14.5 Mo 1.0 23.8 2.6 x 106 4.9 15.4 16.1 Rh 1.o 1.0 x 104 6.0 15.7 Ru 1.0 1.0 x 10s 28.0 8.2 17.7 Pt 1.0 4.6 X lo6 24.0 7.4 15.5 Pd 5.9 x 106 1.0 24.6 6.0 15.2 Co 20.6 5.0 x 105 10.3 1.0 16.3 31.7 CO 8.7 X lo8 4.4 0.1 15.0 Ni 1.5 x 107 1.0 25.9 6.0 15.5 Ni 1.2 x 10: 0.1 4.3 26.0 14.4 Ir 1.0 9.9 x 106 24.9 8.1 16.7 Ir 0.1 1.7 X 10' 5.4 26.2 15.5 Co-Mo 2.0 x 10: 1.0 26.3 5.8 17.2 Ni-Mo 1.4 x 107 1.0 25.8 5.8 18.3 Ni-Mo 0.1 2.8 x 107 4.1 27.2 16.5
(1 - X),,,% 75.0 79.8 81.0 91.6 80.2 78.5 84.4 77.7 80.4 74.5 86.6 79.9 88.7 94.9 85.5
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