Energy & Fuels 1988,2,93-100 of the electrolytic oxidation is to oxidize the kerogen matrix, and it is shown by the abundance of carbonyl and carboxylic matter in the process solution.
Conclusion The dissociation phenomena of the mineral matrix and the mild destruction of kerogen structure in the oil shale system are found to be quite significant. As experimental evidencea show, the associated bitumen is derived from the kerogen through the in situ maturation process. This is proven by the FT-IR spectrum of the oil shale bitumen, where a strong absorption band appeared at 1575 cm-' (an indication for the presence of salts of carboxylic acids, Figure 2), which may have very likely been derived from the mild oxidation of kerogen through the natural geological process. The mild destruction kerogen structure in the described dissociation process can simulate this
93
natural process and derive the final products having properties similar to those of the in situ bitumen. Furthermore, such destruction of kerogen may lead to a new technology for the processing of organic matters as well as the mineral aggregates. However, more studies are needed to better understand this phenomena and the operating variables.
Acknowledgment. We thank John Val Browning, Executive Director of Southern Pacific Petroleum N.L., for providing the Stuart oil shale samples and Drs. C. S. Wen, R. D. Cole, S. Larter, and R. Glassman for all the valuable discussions. Partial support from the donors of the Petroleum Research Fund, administered by the American Chemical Society, is appreciated. Registry No. Montmorillonite, 1318-93-0; illite, 12173-60-3; quartz, 14808-60-7.
Kinetics and Mechanism of Moroccan Oil Shale Solubilization in Toluene? M. Tahiri, C. M. Sliepcevich, and R. G. Mallinson* School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, Oklahoma 73019 Received June 17, 1987. Revised Manuscript Received October 13, 1987
The main objective of this work is to investigate the kinetics and the mechanisms of the solubilization of a Moroccan oil shale kerogen in a toluene solution. The extent of solubilization is defined in terms of tetrahydrofuran extractability in a Soxhlet apparatus. Isothermal kinetic data a t 300 "C obtained by treating pyridine preextracted samples are reported. Gel permeation chromatography (GPC) and 'H NMR were used to investigate changes in molecular weight and structure of the solubilized organic material with treatment time. The experimental data show that about 30% of the originally insoluble organic carbon contained in the shale could be recovered in a mixture of thermally stable molecules of predominently aliphatic structure, having molecular weights in the range 300-2200. Treatment time did not have a marked effect on the elemental composition or on the NMFt spectra of the extracts. On the other hand, the molecular weight distribution exhibited a systematic evolution with treatment time. Analysis of the data revealed that the rate of solubilization of a given species is dependent on its molecular weight. A kinetic model assuming the solubilization to be the result of an infinite set of parallel and independent first-order processes whose kinetic parameters are a function of molecular weight is found to best fit the conversion and molecular weight distribution data. Possible structural features of kerogen and the nature of the mechanisms by which part of it was recovered are discussed.
Introduction Unlike coal, for which solvent liquefaction dates back to the beginning of this century, treatment of oil shale using various solvents has received attention only in recent years as an alternative process to producing liquid fuels by retorting. Solvent extraction offers the promise of overcoming the relatively low yields of desirable products obtained and the high energy expenditures required in pyrolysis processes. Presently the literature abounds with articles dealing with oil shale pyrolysis; only a few articles have appeared 'Presented at the Symposium on Advances in Oil Shale Chemistry, 193rd National Meeting of the American Chemical Society, Denver, CO, April 5-10, 1987.
on solvent treatment of oil shale and even fewer have dealt with mechanistic and kinetic aspects. Many of the articles reporting on solvent extraction mention the enhanced solubility of oil shale's organic matter when contacted with supercritical solvents. However, as shown by the experiments of McKay et aL,l solubilization of kerogen is strongly temperature dependent. Appreciable yields are not achieved until relatively high temperatures (350OC and higher) are used. Clearly at such temperatures, the role of the supercritical solvent in the solubilization process relative to the role of thermal degradation within the kerogen matrix and/or the thermal disruption of organieinorganic bonds has to be addressed. (1) McKay, J. F.; Chong, S.; Gardner, G. W. Liq. Fuels Technol. 1983, I , 259-287.
0887-062~/S8/2502-0093~01.50/00 1988 American Chemical Society
94 Energy & Fuels, Vol. 2, No. 1, 1988
Table I. Oil Shale Analyses Elemental Analysis (% of the Shale) CO, C organics N H
totals 2.68
Tahiri et al.
20.80
17.80
2.42
0.56
ash 58.85
2.22
Fischer Assay (% of the Shale) yield, L/ton
anal.
humiditv
water
6.70
3.40
semicoke gas and losses 87.69
2.16
70
Analysis of the Semicoke of the Fischer Assay (%) CO, total C organic S N H 23.30
10.15
1.20
0.59
0.98
Ash Analysis ( % ) Si02 A1203 Fez03 CaO MgO NazO SO3 Pz05 TiOz 22.34
7.94
3.43
39.20
5.81
0.39
8.11
1.67
0.47
Natural Bitumen Content" (Based on Pyridine Extraction) as a Percent of the Dry Oil Shale 1.4
* 0.1
Determined in this laboratory. On the other hand, kinetic studies on Colorado oil shale2 and on Jordanian oil shale3 concluded that chemical degradation is involved in the thermal solution treatment of oil shale kerogen. Lumped kinetic models for solubilization have been postulated, but the nature of the mechanisms involved has not been determined. To examine the extent of kerogen thermal degradation a t subpyrolysis temperatures in the absence of a solvent, Yucelen et aL4conducted an investigation of Green River oil shale conversion resulting from isothermal heating under a nitrogen atmosphere a t several levels in the temperature range 260-340 "C. Their results showed that kerogen conversion to toluene-soluble and gaseous products occurs even in the absence of a solvent; most of the conversion resulted in gaseous products, however. Those authors reported that although high pressure had no effect on the level of kerogen Conversion, it did shift the selectivity toward production of more toluene-soluble material with a reduction of gas production. McKay and co-workers' also reported that oil shale solvent treatment under supercritical conditions produced very little gas. It is not clear, however, whether this result is due to the high pressures or to some other factor. That oil shale produces, upon heating, a soluble but not volatile material that decomposes upon further heating to condensable and noncondensable volatile products has been established for some time.5i6 This serial transformation of the kerogen has been recently confirmed for Green River oil shale,7for which the formation of an intermediate soluble material was found to be a major pathway of shale oil production from pyrolysis experimenta. Experiments of Cummins and Robinsona showed that, even at temperatures as low as 250 "C, benzenesoluble organics could be produced from Green River oil shale by simple heating. In other words, at temperatures well below those required for carbon-carbon bond cleavage, kerogen can be thermally converted to soluble products (2) Esteves,L. A. Ph.D. Thesis,Universityof Californiaat Davis, 1983. (3) Haddadin, R. A. Fuel 1980,59, 535-538. (4) Yucelen, F.; Smith, J. M.; Wakao, N. AIChE J . 1986,32,607-615. (5) Hubbard, A. B.; Robinson, W. E. Rep. Inuest.-US., Bur Mines 1950, No. 4744. (6) Allred, V. D. Chem. Eng. Prog. 1966, 62, 55-60. (7) Miknis, F. P.; Turner, T. F.; Berdan, G. L.; Conn, P. J. Energy Fuels 1987.1.477-483. (8) Cumm& J. J.; Robinson, W. E. Rep. Znuest.-U.S., Bur. Mines 1972, No.7620.
Table 11. Analyses of the Raw and Pyridine-Extracted Shaler" W C carbonate total organic % N raw dry shale 2.63 14.96 12.33 0.67 pyridine-extracted shale 2.63 14.89 12.26 0.90 " Analyses carried out by Huffman Laboratories, Inc. Table 111. Elemental Analyses" W C % H % N %'S %Ob 1-h extracts 74.81 8.66 3.74 6.27 6.52 5-hextracts 76.16 8.45 2.29 6.32 6.78 natural bitumen 73.08 8.19 3.47 6.60 8.66 "Analyses carried out by Huffman Laboratories, Inc. *By difference. to an appreciable extent. However, the nature of the mechanisms leading to such conversion are not understood. Determination of activation energiesg suggests that such mechanisms involve breakage of physical or weak chemical bonds. Whatever their nature might be, it seems reasonable to assume that the basic mechanisms that form the soluble intermediate in pyrolysis experiments are also operational when oil shales are heated in the presence of a solvent in the same temperature range. The possibility that the use of a solvent may enhance such mechanisms should not be overlooked, however. On the basis of these observations the present study was undertaken to investigate the solubilization of a Moroccan oil shale a t 300 "C and 3000 psig in toluene. In this case the extent of solubilization was measured in terms of tetrahydrofuran extractables a t refluxing conditions. Analyses of the soluble material produced were obtained by gel permeation chromatography and proton NMR.
Experimental Section Oil Shale Sample. The oil shale specimen used in this investigation was obtained from the YFO stratum of the Timahdit formation (Morocco),courteey of the office National de Recherche et d'Exploitation du Petrole (ONAREP). Analyses supplied by the ONAREP are given in Table I. The organic carbon, carbonate carbon, and the nitrogen contents of the raw shale are shown in Table 11. Sample Preparation. Raw oil shale particles, 0.5 cm in size, were ground to -100 mesh. A 300-g sample was exhaustively Soxhlet extracted by using pyridine at its normal boiling point until the refluxing solvent became clear, which required about 1 week. The objective was to remove the organic matter, commonly referred to as natural bitumen, from the oil shale. The extracted shale was then dried under 25 in.Hg vacuum at 105 "C for 24 h and for an additional2 h at 125 "C.The shalewas allowed to cool in a dessicator for 1 / 2 h, after which it was subdivided, via a standard sampling technique, into 20 samples of about 15 g each. These individual samples were dried for 1 h at 125 "C under vacuum, cooled in a deacicator,and weighed. Since these samples do not contain bitumen or moisture,they will be referred to as dry, bitumen-free shale (DBFS). The organic carbon, carbonate carbon and the nitrogen contents of the DBFS are shown in Table II. There were indications that even after vacuum drying some pyridine remained incorporated in the shale; indeed the odor of pyridine could be detected from the shale. The material balance on the pyridine-extractedshale showed that the extent of pyridine incorporation amounted to about 1.3% on a dry shale basis. Further corroboration of the pyridine incorporation in the extractedshalewas obtained h m the analyses shown in Table I1 where the nitrogen content of the extracted shale is seen to be higher than that of the raw shale. A nitrogen balance, taking into account the amount of bitumen extracted and its (9) Braun, R. L.; Rothman,A. J. Fuel 1975,54, 129-130.
Energy & Fuels, Vol. 2, No. 1, 1988 95
Moroccan Oil Shale Solubilization nitrogen content shown in Table 111, showed that the extent of pyridine incorporation in the extracted shale is about 1.6%. Apparatus. The experimental apparatus consists of a stainless-steelreaction tube suspended in a fluidized sand bath. The sand bath temperature was monitored with a type K thermocouple and was controlled by a solid-state digital temperature controller. type The inside reactor temperature was monitored using a l/l&. K thermocouple and the inside reactor pressure by a 6000 psig gauge. The reactor has an internal diameter of 1.75 cm and a length of 17.5 cm for a total volume of about 47 cm3. High-Temperature Experiments. In a typical run, about 15g of DBFS and 40 cm3of HPLC grade toluene were introduced into the reactor, which was sealed. The sand bath was initially heated to 330 "C based on preliminary runs, which showed this value to be optimal for minimizing the heatup period. The reactor was immersed into the sand bath and was shaken continuously. About 4 min were needed for the reactor temperature to reach 300 "C after which it stabilized to within k2 "C for the entire experiment. For runs 30 min and longer the reactor pressure stabilized at 3000 100 psig. This pressure was achieved by adjusting the quantity of solvent in the reactor; no gaseous pressurization was used. Once a run was finished, the reactor was removed from the sand bath and quenched in cold water. In less than 1min the reactor temperature was reduced to less than 60 "C. The pressure dropped to 0 psig when the room temperature is achieved within the reactor. In other words, no residual pressure was detected in these experiments,suggesting that the amount of gas make is negligible. At the end of a run the reactor was opened, and its contents were emptied into a beaker with toluene used as a wash solvent. The system was then flushed with toluene, and the shale plus toluene solution was filtered into a paper thimble, the contents of which were subsequently Soxhlet extracted with THF at its normal boiling point. About 12 h were needed for the refluxing solvent to become colorless. The Soxhlet extracts and the reactor filtrate were combined after which the solvents were evaporated by using a rotary evaporator under reduced pressure. The solution volume was reduced to less than 15 mL. The remaining solvents were evaporated at 80 "C and 25 in.Hg of vacuum for 12 h and at 105 "C for an additional 12 h, or as needed, until the weight of the extractedmatter became constant. The combined extracts were then cooled in a desiccator and weighed, and the percentage of total extractable8 on a dry bitumen-free basis was determined according to w t of dried extracts % conversion = (1) wt of DBFS Gel Permeation Chromatography Analyses. Samples of the
extracts were analyzed for their molecular weight distribution by using a Waters GPC I instrument equipped with a differential refractometer. Three Ultrastyragel columns of 100,500,and lo00 8, were used to achieve an appropriate separation. The solvent used was HPLC grade THF at a flow rate of 0.9 mL/min. The temperature in the columns was maintained at 32 "C. The GPC was interfaced with an Apple IIe computer using Interactive Microware's Chromatochart software for data aquisition and analysis. Proton NMR Analyses. The proton NMR analyses were performed on an IBM NR 80 Fourier transform instrument. Deuteriated chloroformwas used as solvent and tetramethylsilane as the reference.
Results and Discussion Elemental Analysis and Proton NMR Data. Elemental analyses of the natural bitumen and of the extracts obtained after 1-h and 5-h treatment time at 300 "C are shown in Table 111. These data show that the extracted organic matter has a high heteroatom content, especially sulfur and oxygen. The results also show that there is no significant change of the elemental composition with treatment time. Furthermore, natural bitumen seems t o have a higher oxygen content than the organic matter recovered at high temperature. Proton NMR data showed little difference with treatment time, indicating that all of the extracts have very
Figure 1. 'H NMR spectrum for the extracts obtained after 20-min treatment time.
4 0'
3 5; m 3 01
0
60
120
A
GLOBAL
O
LOW
*
MEDIUM MW FRACTION
0
HIGH
180
MWFRACTION
,
MW FRACTION
240
300
360
TIME (MIN)
Figure 2. Conversion as function of treatment time at 300 "C. similar structure. Although some differences were observed in peak intensities, no quantitative trend was observed. Figure 1 shows the spectrum of the extracts obtained from the experiment at 20 min. The structure of the extracted organic matter recovered under the experimental conditions of this study was observed t o be basically aliphatic. This was somewhat unexpected since solid-state 13CNMR datalo have suggested that the organic matter of the Moroccan oil shale has an aromaticity of about 0.4. However, only a fraction of the insoluble organic matter has been recovered in our experiments; this, coupled with the fact that aromatic moieties of kerogen are known to be more difficult to recover, might explain this difference. As will be discussed shortly, more organic material becomes soluble with increased treatment time up to an asymptotic limit. The lack of significant change in the structure and composition of this material implies that as time increases, more of the same material becomes soluble. This also implies that the recovered organics are not exposed to conditions under which they would be thermally degraded. These observations seem to be in agreement with McKay's assesmentll that the chemical composition of the organic material is essentially constant whenever it is extracted without thermal alteration from the oil shale. Conversion Data. The data for conversion to THFsoluble material, as a percentage of the dry bitumen-free (10) Miknis, F. P.;Szeverenyi N. M.; Maciel G. E. Fuel 1982, 61, 341-345. (11) McKay, J. F.Energy Sciences 1984, 7, 257-270.
96 Energy & Fuels, Vol. 2, No. 1, 1988
Tahiri et al.
shale, are shown in Figure 2. The point a t zero time corresponds to a run that was terminated when the reador temperature reached 300 OC. The data show the conversion to increase with time but to tend to an asymptotic value of 5%. These data are on a DBFS basis, which as we mentioned earlier, has some pyridine incorporated in it. If this pyridine is taken into account, the figures would be slightly higher than those presented in Figure 2, but the ensuing correction is small compared to the experimental uncertainties. The best indication of the extent of conversion in these experiments is the percent of the originally insoluble organic matter recovered as T H F solubles; however, the amount of the total organic matter in the raw shale is not known. As an alternative, the determination of the extent of conversion is based upon the organic carbon contents of the DBFS and of the extracted material. After 5-h treatment time, 30% (33% if incorporated pyridine is taken into account) of the organic carbon of the dry bitumen-free shale is converted to THF-soluble material. In these experiments the overall mass balance was deficient by from 1to 3% on a DBFS basis. We believe that most of this deficit is due to the release of the retained pyridine after the 300 "C toluene treatment. The fact that some of the light organic compounds may be lost in the process of separating the extracts from the process and extraction solvents and the possibility of some gas make, however small, are factors that may contribute to the deficiency of the material balance. On the basis of the examination of the data of Figure 2, the following nth-order kinetic model for the percent conversion P ( t ) a t time t was postulated:
In the rate expression (eq 2)) K, is the rate constant and P, is the ultimate conversion a t long processing times. It is to be emphasized that no mechanistic interpretation is attached to the parameters appearing in (2), which merely states that the "driving force for conversion" is the remaining unconverted organic matter and that the conversion tends to an asymptotic value, P,, as t a. When the rate expression is integrated from time 0 to an arbitrary value t, the following expressions for the percent conversion are obtained:
-
for n # 1 1J.
(P, - P(t))"l
-
1A.
(P,- P0)n-l
= (n - l)Knt
(3)
for n = 1 (i.e. the first-order model)
P(t) = P, - (P, -Po)exp(-K,t)
(4)
In the above equations, Pois the percent conversion a t time 0; experimentally, it is what is solubilized during the heatup period. A standard nonlinear regression routine was used to fit (3) to the experimental data. The value obtained for the parameter n was 0.99, suggesting a first-order model. Linear least-squares regression of In (P, - P ( t ) versus time, where P, was assigned the experimental value 5.0, gave K = 0.0094 min-' and Po = 0.95 with a correlation coefficient, r = 0.995. The solid line through the data shown in Figure 2 is based on (4) with the aforementionedparameters. It shows that a first-order model does indeed correlate the experimental data very well. However, it would be premature to attach a mechanistic significance to such a model.
251 0
0
g20. X
z
___ ___ _ __ _ ___ _ ________ ....................
0 MIN 20 WIN 60 MIN l2OMIN 240 MIN 360 MIN BITUMEN
0
Molecular Weight Analysis Data. GPC analyses showed natural bitumen to have a number-average molecular weight of 580 with the breadth of the distribution ranging from 350 to 1800. The complete molecular weight distribution for bitumen is shown in Figure 3. McKay et al.' reported an average molecular weight by vapor-pressure osmometry between 500 and 600for bitumen of Green River oil shale. Molecular weight distributions for some of the extracts obtained a t different reaction times are also shown in Figure 3. The molecular weight range for the recovered organics extends from 300 to 2200. By examination of Figure 3, the following observations can be made: As the reaction time increases, increasing amounts of larger molecules are recovered. A sharp peak is observed around 420 amu for short reaction times; however, its relative intensity diminishes as the reaction time increases. There is a smooth progression in the molecular weight distribution with reaction time. For reaction times of 3 h and beyond, little change occurs. For 5 h (not shown for the sake of clarity) and 6 h the molecular weight distributions are essentially identical. In order to study the effect of reaction time on the percentage recovery of different molecular weight fractions the molecular weight range was subdivided into three fractions: low molecular weight fraction (MW < 600); medium molecular weight fraction (600 < MW < 1200); high molecular weight fraction (1200 < MW < 2100). The delineation of three regions of molecular weight was essentially arbitrary; it was arrived a t by noting the differing qualitative behavior in these molecular weight ranges. Production of the different molecular weight fractions as a function of time is shown in Figure 2. The increases of each molecular weight fraction with time are significant in that it c o n f i i that the species produced are stable and indicates that no extensive pyrolysis (in the sense of carbon-carbon bond cleavage) occurs under the experimental conditions investigated. The conversion data of Figure 2 show that the percentage recovery of each molecular weight fraction approaches an asymptotic value estimated to be 1.65, 2.35
Energy & Fuels, Vol. 2, No. 1, 1988 97
Moroccan Oil Shale Solubilization
*
0 251
o,2 1
0.20
0.05
0
MW FRACTION 3 4 8 - 3 7 8
*
MW FRACTION 6 1 3 - 6 6 3
0
MW FRACTION 1 1 5 8 - 1 2 5 8
~
0
OOO,,
,
,
,
0
,
I
,
,
,
,
,
I
,
120
60
, , , , , , , 180 240 TIME (MIN) ,
,
,
I
,
,
,
, ,
,
, , ,
300
,
,
700 OC because of the decomposition of NH3. The NH3 yield showed no dependence on the heating rate a t PT < 550 OC and decreased with increasing heating rate at higher peak temperatures. For Green River Formation shales, the yield of NH3 a t PT < 500 "C reflected the organic nitrogen content. The extent of NH3 decomposition varied with gas environments, solid surface, and the conditions of the retort can. Because steam works as an inhibitor for the decomposition reaction, we used steam as a retorting gas to obtain the total yield of NH3. We developed a kinetic model for NH3 evolution; it takes into account NH3 generation from organic and inorganic nitrogen sources and NH3 decomposition.
-
Introduction Nitrogen (N) species such as ammonia (NHJ, NO,, and HCN can be released during the combustion of fossil fuels. Various laws limit their emissions because of environmental concerns, so the study of N species is of particular importance in oil shale processing because both oil shale and shale oil are rich in N. Ammonia is the major nitrogen species evolved during oil shale processing, where it is found in retort water, in retort off-gas,l and in the combustor gas with NO,. During combustion, NH3 is also important because it is an intermediate in conversion of fuel N to NO, and NP Effective control of NO, emission requires a better understanding of the formation and decomposition of NH3 during combustion. As early as 1865, ammonia had received a lot of attention because it was a valuable by-product of shale processing.2
* To whom correspondence should be addressed.
t Work performed under the auspices of the U.S.Department of Energy by the Lawrence Livermore National Laboratory under Contract No. W-7405-Eng-48.
*Presentedat the Symposium on Advances in Oil Shale Chemistry, 193rd National Meeting of the American Chemical Society, Denver, CO, April 5-10, 1987.
0887-0624/88/2502-0100$01.50/0
The major conclusions from early work were that (1)a large fraction of NH, is produced at temperatures higher than oil-evolving temperatures, (2) the yield of NH, is enhanced in steam retorting, and (3) the yield of NH3 is also enhanced a t slower heating rates., In this study, we investigated ammonia evolution along with the changes in the N distribution among pyrolysis products a t the different stages of oil shale pyrolysis. First, we looked at the effects of pyrolysis temperature, heating rate, N content, and the form of N in raw shale on total NH3 yield. Then, we measured the continuous rate of ammonia evolution during pyrolysis and also studied the extent of NH3 decomposition a t high temperatures and its effect on total NH3 yield.
Experimental Section We conducted three different types of experimentsto measure (1)the total yield of NH, at a given time-temperature history, (2) the rate of ammonia evolution as a function of time during (1) Sklarew, D. S.; Hayes, D. J., Enuiron. Sci. Technol. 1984,18,600. ( 2 ) Bell, H. S. Oil Shales and Shale Oils: Van Nostrand: New York, 1948.
0 1988 American Chemical Society
