Effect of temperature on the kinetics of kerogen ... - ACS Publications

206

Energy & Fuels 1989, 3, 206-215

Effect of Temperature on the Kinetics of Kerogen Solubilization for a Moroccan Oil Shale in Toluene M. Tahiri,t C. M. Sliepcevich, and R. G. Mallinson" School of Chemical Engineering and Materials Science, University of Oklahoma, Norman, Oklahoma 73019 Received October 7, 1988. Revised Manuscript Receiued November 7, 1988

The objective of this work is to extend our previous investigation1on the kinetics and the mechanisms of the solubilization of the organic matter of a Moroccan oil shale in toluene at 300 "C to 320 and 340 "C. Isothermal conversion to soluble material is based on extractability by tetrahydrofuran (THF) in a Soxhlet apparatus. Elemental analysis, gel permeation chromatography (GPC), and proton nuclear magnetic resonance spectroscopy (lH NMR) were used to investigate, respectively, changes in composition, molecular weight, and structure of the solubilized organic material with treatment time and temperature. The experimental data showed that asymptotic conversions of 38% and 44%, based on the organic carbon content of the shale, were achieved at 320 and 340 "C, respectively. The extracted organic material exhibited some loss in oxygen content with treatment time and temperature. However, the effect of these parameters on the elemental composition was not drastic; the average composition for the extracts was 76% carbon, 8% hydrogen, 7% oxygen, 3% nitrogen, and 6% sulfur. The structures of the extracts were predominantly aliphatic and were not markedly affected by either time or temperature. On the other hand, the molecular weight distributions of the extracts exhibited substantial changes with both time and temperature. In particular, it was found that while the compounds recovered were thermally stable at 300 and 320 "C, the compounds with a molecular weight above 1070 were found to undergo thermal degradation reactions at 340 "C to yield smaller species. Furthermore, an analysis of the experimental data showed that the recovery of these high molecular weight species involved activation energies sufficiently high to suggest that chemical bonds were broken to solubilize them. On the other hand, the activation energies involved in the recovery of compounds with smaller molecular weights were low, which suggests a diffusion-limited mechanism for their Solubilization. In light of the experimental data, possible structural features of the kerogen of this particular oil shale are discussed.

Introduction The disturbances in crude oil supplies of the past decade and the ensuing concerns over the availability, security, and pricing of this strategic resource have stimulated intensive research pertaining to liquid fuels production from oil shale. The bulk of these efforts have focused on retorting technologies, which invariably involve destructive distillation of the organic matter of oil shale at temperatures typically above 500 "C. However, high temperatures require substantial energy expenditures and result in an inefficient utilization of the organic matter of the shale due to cracking and coking reactions by generating substantial amounts of products having a low heating value. Because of these limitations, there has been increasing interest in alternative technologies for oil shale processing based on mild thermal treatments in the presence of solvents. There seems to be general agreement that higher yields can be obtained by using solvents when compared to dry retorting method^.^-^ However, some caution has to be exerted when comparing yields from dry retorting and solvent experiments; the former are based on volatile products and the latter on soluble products. Miknis et a1.6 have, for example, shown that yields as high as 95% organic carbon conversion can be obtained from retorting a Colorado shale without a solvent under appropriate conditions. This yield is in the range of those reported by McKay et a1.2 from water-methanol autoclave experiments on a Colorado shale and by Leavitt et a1.8 from tetralin autoclave experiments Present address: Ecole Mohammadia d'IngCnieurs, Rabat, Morocco. 0887-0624/89/2503-0206$01.50/0

on a Utah oil shale. Clearly, then, the advantages (or lack thereof) of using solvents for oil shale treatment cannot be assessed solely on the basis of the highest yields obtainable. Such questions ought to be answered on the basis of the mechanisms and the kinetics involved as well as the quality of the products obtained. Investigations in these areas have been less common, and the knowledge available at this time is somewhat fragmentary. McKay et al.2*3investigated the recovery of organic matter from Green River oil shale under mild thermal conditions (300-400 "C) using various solvent systems. They achieved a 90% recovery of the organic carbon in the shale as a form of liquid organic material using a watermethanol solvent at 400 "C. At the same temperature they obtained a 70% yield by heating the shale under an argon atmosphere. The authors reported evidence of coke formation in the latter experiment. McKay and co-workers reported that the composition of the organic matter recovered in the water-methanol experiments was not significantly affected by temperature even though the yield (1) Tahiri, M.; Sliepcevich, C. M.; Mallinson R. G. Energy Fuels 1988, 2, 93-100. (2) McKay, J. F.; Chong, S.; Gardner, G. W. Liq. Fuels Technol. 1983, 1 , 259-287. (3) McKay, J. F.; Chong, S. Liq. Fuels Technol. 1983, I , 289-324. (4) Baldwin, R. M.; Chen, K. W. Fuel 1987, 66, 353-357. (5) Yurum, Y.; Kramer, R.; Levy, M. Thermochim. Acta 1986, 105,

__

51-62.

(6) Miknis, F. P.; Turner, T. F.; Berdan, G. L.; Conn, P. J. Energy Fuels 1987, I , 477-483. (7) Haddadin, R. A. Fuel 1980,59, 535-538. (8) Leavitt, D. R.; Tyler, A. L.; Kafesjian, A. S. Energy Fuels 1987, 1 , 520-525.

0 1989 American Chemical Society

Energy & Fuels, Vol. 3, No. 2, 1989 207

Kinetics of Kerogen Solubilization

increased with higher temperatures. However, they reported that at 400 "C the organic matter experienced some oxygen loss and a reduction in molecular weight. These authors suggested that although the operational mechanisms in their experiments are poorly defined and may encompass such events as bonds and linkages cleaving within the organic matrix or desorption of strongly adsorbed organic compounds from mineral surfaces, an important mechanism may involve disintegration of the shale matrix whereby inorganic-organic interactions are disrupted, allowing the recovery of organic compounds. More recently Chong and McKaf reported that methanol reacts with the carboxylic acids that are known to be indigenous to Green River oil shale, to form methyl esters. This result suggests, according to the authors, a decrease in the interactions between the organic compounds of the shale and its mineral carbonates and hence an easier extraction of the organics in the experiments where the oil shale is treated with a water-methanol solvent system. Yurum et al.5 investigated thermal effects in the interaction of an Israeli oil shale with supercritical toluene at 340 "C. Their differential scanning calorimetry (DSC) data showed the existence of two endothermal effects at 200 and 300 "C and no thermal effects even after long periods of treatment time a t 340 "C. When the same oil shale was heated without toluene, DSC data also showed two endothermic effects, but at the much higher temperatures of 380 and 485 O C . Yurum and co-workers concluded that the endothermal effects observed with and without supercritical toluene arise from bond cleaving in the kerogen structure. Such endotherms could, in principle, be due to "phase" transitions, which might enhance the mobility of "unpolymerized" portions of the kerogen. They suggested that toluene lowers, by some unexplained chemical mechanism, the activation energies of pyrolysis reactions that would otherwise require higher temperatures. Baldwin and Chen4 observed that the presence of toluene at 450 "C enhanced carbon conversion to soluble forms for two Australian oil shales. EsteveslO studied the rates of Green River kerogen solubilization in supercritical toluene. He reported that a model based on extraction alone could not fit his experimental data, and he concluded that a chemical step was involved in the solubilization process. Triday and Smith" confirmed this conclusion and suggested that the thermal decomposition of kerogen to soluble products is enhanced by the presence of supercritical toluene via an activated solvation process. However, the nature of this mechanism was not detailed. Leavitt et aL8obtained nonisothermal kinetic data from the solubilization of kerogen in thermal tetralin solutions for a Green River (Utah) oil shale and a more aromatic Sunbury (Kentucky) oil shale. Up to 95% and 75% conversions to soluble organic material were obtained for the two shales, respectively. A parallel first-order model was found to fit the data from the Green River shale with the computed activation energies of 20.8 and 45.7 kcal/mol. The authors interpreted this result as meaning that the solubilization of kerogen for this oil shale involves breaking of weak chemical or physical bonds at temperatures below 350 "C and stronger chemical bonds a t higher temperatures. The data from the Sunbury shale suggested, according to the authors, a serial mechanism with an activation energy of about 50 kcal/mol for the second step. Another contrast between the data for the two oil shales (9) Chong, S.; McKay, J. F. Fuel Sei. Technol. Int. 1987,5, 513-541. (10) Esteves, L. A.PhD Dissertation, University of California at Davis,

1983.

(11) Triday, J.; Smith, J. M. AIChE

J. 1988, 34, 658-668.

Table I. Analyses of t h e Raw and Pyridine-Extracted Shales %

carbonate % % % C total C organic C nitrogen raw dry shale 2.63 14.96 12.33 0.67 2.63 14.89 12.26 0.90 pyridine-extracted shale

concerns the elemental composition of the organic matter left in the spent shale. This composition was quite constant for the Green River oil shale throughout the investigated temperature range, whereas it changed significantly for the Sunbury oil shale. For the latter, the aromatic carbon and nitrogen increased and the hydrogen decreased a t temperatures above 400 "C. For lower temperatures, a significant depletion of hydrogen, oxygen, and sulfur from the parent kerogen was observed. Yurum et a1.12 studied the kinetics of an Israeli oil shale solubilization in toluene at 340 "C. They observed a rapid increase in the conversion to soluble material in the first 60 min, after which it approached an asymptotic value of about 60% of the total organic content of the shale. The GPC and FT-IR analyses showed an increase in the molecular weight of the solubilized material and no change in its structure during the first hour. Subsequently, a decrease in the molecular weight concomitant with an increase in aromaticity and a decrease in the hydroxyl group content was observed. Yurum and coinvestigators attributed the disintegration of the material to lower molecular forms that occurred at extended times under supercritical, isothermal conditions to a "loosening" of aggregates of molecules that were originally bound together with weak interactions. Clearly much work is needed toward a better understanding of the kinetics and the mechanisms of the solubilization of the organic matter of oil shale in solvent media. The present work is an attempt to contribute to this understanding by investigating the conversion of the kerogen of a Moroccan oil shale to THF extractables upon heating in toluene. The extracted organics have been analyzed by GPC, 'H NMR spectroscopy, and elemental analysis to examine how these properties are affected by time and temperature. The elemental analyses for some residues were also carried out. Results at one temperature, reported in an earlier paper,' were encouraging and showed that some insight could be achieved, not only about the kinetics and mechanisms but also about the structure of the kerogen.

Experimental Section The experiments and analyses of this study are identical with previous work,' except for the higher temperatures used. Thus, only a very limited description is provided. The oil shale specimen used in this investigation was obtained from the YFO stratum of the Timahdit formation (Morocco), courtesy of the Office National de Recherche e t d'Exploitation du Petrole (ONAREP). Detailed analyses supplied by the ONAREP are given elsewhere.'J3 The organic carbon, carbonate carbon, and the nitrogen contents of the raw shale are shown in Table I. A detailed description of the experimental apparatus is given e1~ewhere.I~ The reactions were carried out in a 47 cm3 stainless-steel reaction tube suspended in a fluidized sand bath. The amount of toluene in the reactor was adjusted to obtain the desired pressure a t a given temperature by a hand pump to maintain a constant density. In a typical run about 15 g of DBFS and 40 cm3 of toluene were introduced into the reactor, which was sealed. (12) Yurum, Y.; Kramer, R.; Levy, M. Fuel Sci. Technol. Int. 1986,4, 501-529. (13) Tahiri, M. PhD Dissertation, University of Oklahoma, Norman,

OK,1988.

Tahiri et al.

208 Energy & Fuels, Vol. 3, No. 2, 1989 Table 11. Elemental ComDosition of the Extracts (wt %) composition hydrogen oxygen nitrogen 300 OC' 8.66 6.52" 3.74 8.45 6.78O 2.29

treatment time, min

carbon

60 300

74.81 76.16

0 20 60 120 300

73.41 75.01 75.78 74.14 77.51

8.16 8.23 8.16 8.18 8.10

320 "C 9.33 8.03 7.35 6.17 6.01

20 30 41 60 60 90 120 180 180 240

75.87 75.36 76.73 76.54 76.94 77.28 78.02 77.57 78.52 78.52

7.99 8.11 8.05 7.93 8.02 7.95 8.11 7.97 8.11 8.05

340 "C 7.39 7.01 6.10 6.96 5.64 6.72 6.23 6.99 4.05 5.36

sulfur

(H/ C) .trim

6.27 6.32

1.39 1.33

2.96 2.72 2.52 2.47 2.71

5.74 6.31 6.61 6.12 6.16

1.33 1.32 1.29 1.32 1.25

2.57 2.71 2.63 2.57 2.66 2.76 2.66 2.73 2.61 2.70

6.69 6.13 6.82 6.15 6.89 5.03 5.40 5.11 6.11 5.62

1.26 1.29 1.26 1.24 1.25 1.23 1.25 1.23 1.24 1.23

Data for Green River Oil Shale from Reference 3 comDosition ~~

temp, "C 300 350 375 400

convn; % 13.4 26.6 52.3 89.5

carbon 78.4 79.8 79.0 80.4

hydrogen 10.8 11.0 10.6 10.5

oxygen 6.6 5.8 5.4 4.7

nitrogen 1.3 1.6 1.9 2.6

sulfur 1.1

b 1.0 0.5

W/CLtm 1.65 1.65 1.61 1.57

"By difference. *Not determined. ePercent of the organic carbon converted to soluble material. No gaseous pressurization was used. No residual pressure was detected in these experiments,suggesting that the amount of gas make was negligible. The percentage of total extractables on a dry bitumen-free basis was determined according to wt of dried extracts % conversion = x 100 (1) wt of DBFS

Samples of the extracts were analyzed for their molecular weight distribution by using a Waters GPC I with three ultrastyragel columns using THF as the solvent. The proton NMR analyses were performed on a Varian XL-300 instrument. Analyses for the carbon, hydrogen, oxygen, sulfur, and nitrogen for the organic extracts and the spent shale of selected experiments were performed by Huffman Laboratory, Golden, CO.

Results and Discussion Elemental Analysis and Proton NMR Data. The elemental analysis data for the extracts are shown in Table 11. Data from the previous work' at 300 "C are also included for comparison. The percentage of carbon in the 320 and the 340 "C extracts show a slight increase with treatment time at either temperature. Also, the 340 "C extracts obtained after a given treatment time have a slightly higher carbon content than the 320 "C extracts. However, these trends are not dramatic. The data show that the hydrogen, nitrogen, and sulfur contents of the 320 "C extracts are fairly constant. The oxygen content in these extracts, on the other hand, shows a decreasing trend during the first 2 h of treatment time. Such a decrease may be due to a loss of oxygen by chemical reactions from the extracts. A decrease in the oxygen content of watermethanol extracts from a Colorado shale, as the treatment temperature was raised to 400 "C, could have resulted from decarboxylation reactions of some oxygen-containing organic functionalitie~.~,~ However, in the present study the decrease in the oxygen content of the extracts was concomitant with an increase in the amount of organic matter recovered from the shale. Consequently, the possibility

of recovery of more material containing less oxygen might be responsible for the observed decline. The concentrations of hydrogen and nitrogen in the 340 "C extracts are fairly constant. The data at 340 "C seem to indicate a decrease in the oxygen concentration. The concentration of sulfur in the extracts at this temperature also seems to decrease during the fiist hour of treatment time. Contrary to the data pertaining to hydrogen and nitrogen, which exhibited good reproducibility, there is some scatter in the oxygen and the sulfur data. In particular the concentrations of these two elements in the 60- and the 180-min extracts showed poor reproducibility. The duplicate analyses were conducted for samples of the same extracts but after a time interval of about 15 days. It is not clear whether the discrepancies are due to experimental errors or to an actual change in the composition of the extracts in the time between the first and the second analyses. In general, it can be concluded that the effects of both time and temperature on the elemental composition of the extracts are not substantial except for oxygen whose concentration in the extracts seems to decrease with treatment time. The elemental composition data in the present work show that the organic matter in the Moroccan oil shale has a higher heteroelement content, especially sulfur, than the organic matter from a typical Colorado oil hale.^^^ Some solid residue samples were analyzed for their content in organic carbon, carbonate carbon, oxygen, hydrogen, nitrogen, and sulfur. The results of these analyses are shown in Table 111. The data show a depletion of organic carbon from the 320 and the 340 "C residues. This trend is consistent with the extraction of increasing amounts of organic material with treatment time, as will be discussed subsequently. The data show decreases in the concentrations of oxygen, hydrogen, and sulfur from the 320 "C residues; these trends are also consistent with the extraction of increasing amounts of organic material from the shale. The nitrogen data do not exhibit a similar



Table 111. Elemental Composition of Residues (wt %) composition treatment H 0 N S carbonate C time. min organic C Raw, Moisture-Free Oil Shale 12.33 0.67 2.63 Pyridine-Extracted Oil Shale (DBFS) 12.26 0.90 0 20 60 120 240 300

10.39 9.95 9.08 8.10 7.50 7.35

30 90 240

7.79 6.58 6.01

320 OC 1.49 14.54 1.37 14.38 1.27 13.92 1.19 13.59 1.07 13.95

0.64 0.59 0.56 0.58 0.53 0.63

3.03 2.97 2.85 2.67 2.74

2.63 2.71 2.74 2.67 2.76 2.88 2.68

340 OC 2.87 2.88 2.88

Table IV. Organic Carbon Conversion and Removal (wt % ) As Computed from the Organic Carbon Contents of the Extracts and Residues treatment time, min % occ % OCR % OCR - % OCC 320 OC 0 10.84 16.79 5.95 20 16.03 20.97 4.94 60 25.71 29.02 3.31 120 32.84 37.52 4.68 240 36.71a 42.44 5.73 300 37.74 43.63 5.89 340 OC 20 30 41 60 90 120 180 240

29.52 32.52 35.42 39.21 41.48 42.76 44.00 44.45

b 39.82 b b 49.86 b b 54.38

7.30 8.38 9.84

a The carbon content of the extract from this run was not determined; the value computed was based on an interpolation of the data of Figure 1. Residues of these runs have not been analyzed for their elemental composition.

I 0

60

120 180 240 TREATMENT TIME (MIN)

300

360

Figure 1. Overall conversion at different temperatures.

tendency to decrease, which may be due to the fact that a substantial amount of nitrogen in the shale is associated with organic matter that could not be extracted under the experimental conditions used. (Nitrogen-containing organic compounds from Green River oil shale were found to be difficult to The concentration of carbonate carbon in the 320 and the 340 "C residues do not decrease with treatment time, indicating that decomposition of mineral carbonates did not occur to a detectable extent under the conditions of these experiments. Proton NMR data showed little difference with treatment time or temperature, indicating that all of the extracts have very similar carbon-hydrogen structures. Although some differences were observed in peak intensities, no quantitative trends were observed. The structure of the extracted organic matter recovered under the experimental conditions of this study was observed to be primarily aliphatic. Conversion Data. The data for conversion to THFsoluble material, as a percentage of the dry bitumen-free shale (DBFS), me shown in Figure 1. Data at 300 OC, from previous work: are included for comparison purposes. The points a t zero time correspond to runs that were terminated when the reactor temperature reached the temperature of the isotherm. Conversion increases with time a t all temperatures, approaching asymptotic values of about 5% at 300 "C, 6% at 320 "C, and 7% at 340 "C.

These data are on a DBFS basis, which, as mentioned earlier, has some pyridine incorporated in it. If this pyridine is taken into account, the percentages would be slightly higher than those presented in Figure 1. 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 can be based upon the organic carbon content of the DBFS and of the solid residue and the carbon content of the extracts. Thus the percentage of organic carbon conversion to soluble material (% OCC) and the percentage of organic carbon removal (% OCR) from the shale by extraction are given in Table IV. These were calculated from eq 2 and 3. In these equations e is the percentage of extracts on %

occ = -

% OCR = 100 X

dC

d, - r,(l - e ) dc

(3)

a DBFS basis, e, is the percentage carbon in the extracts, and dc and r, are the percentages of organic carbon in the DBFS and residue, respectively. Equation 3 is based on the assumption that there are no losses; i.e., DBFS = extracts + residue. That this assumption is not completely correct can be seen by the examination of the last column of Table IV, which gives the difference between the percentage conversion of the organic carbon to extractable forms and the percentage of its removal from the shale, indicating that some organic carbon is not accounted for. As mentioned before, some pyridine was retained by the shale in the process of natural bitumen extraction and possibly was released during the toluene treatment and eliminated from the extracts during drying. The release of retained pyridine represents a loss of organic carbon that is not accounted for in the organic carbon balance. The hypothesis of pyridine release is supported by the fact that the percentage of nitrogen in the residues, 0.6% on average, is well below the percentage of nitrogen in the DBFS, Le., 0.9%. Another possible source for loss of some carbon is through the evaporation of low molecular weight compounds during the drying of the extracts. Gas production

Tahiri et al.

210 Energy & Fuels, Vol. 3, No. 2, 1989 Table V. First-Order Kinetic Parameters at Various Temperatures P,, % of Po,% of temu, "C K. m i d DBFS DBFS 300 320 340

0.0092 0.0146 0.0246

5.04 6.04 6.94

0.98 1.68 3.62

--

___ ___ _ ___ ___ _ ________

015' 0

0

....._..............

2 does not occur to an appreciable extent in the experiments since, as mentioned before, no detectable residual pressure was observed when the reactor was quenched to room temperature. Furthermore, any gas production would be expected to increase with treatment time, but the carbon loss shown in Table IV does not show such a trend at 320 "C. At 340 "C the few data available seem to indicate some increase in carbon loss with treatment time. As will be discussed subsequently, the molecular weight data analysis indicate that low molecular weight compounds were produced through the thermal degradation of the extracts at 340 "C. Some of these low molecular weight compounds conceivably were lost by evaporation, which explains why the loss of carbon at 340 "C is higher than at 320 "C and increases with treatment time. When a simple power law model is used, the data of Figure 1 were found13 to be best fitted by a first-order model, i.e. P ( t ) = P, - (P, -Po) exp(-Kt) (4) In eq 4, P ( t ) is the percent conversion on a DBFS basis at time t. Po is the percent conversion at time 0; experimentally it represents the amount solubilized during the heatup period. P,, is the ultimate conversion and K is the first-order kinetic parameter. The three adjustable parameters of (4) were determined by a standard nonlinear regression routine, and the values are given in Table V. The absolute average deviation with these parameters was less than 4% in all cases. The lines through the data shown in Figure 1 are based on the first-order model fit. The temperature dependence of the kinetic parameter K was found to follow Arrhenius type behavior over all three temperatures. The activation energy is about 17 kcal/(g mol) and the frequency factor about 30000 m i d . A good fit of the experimental data and the fact that the kinetic parameter is well described over the relatively narrow temperature range used in this work do not constitute validation of a first-order model. Earlier work' has shown that the actual mechanism is, in fact, more complex. The data of Figure 1 are a further confirmation of this complexity since the ultimate conversion is temperature dependent, which is inconsistent with a true first-order model. Molecular Weight Data Analysis. Molecular weight distributions for the extracts obtained at 320 "C are shown in Figure 2. Examination of these data shows that the molecular weight distribution for the extracts obtained at 320 O C are similar to the ones obtained for the 300 "C extracts.' These data show a gradual shift of the molecular weight distribution toward higher molecular weights, suggesting recovery of increasing amounts of larger molecules with treatment time. The sharp peak in the low molecular weight region observed for the short time extracts diminishes in intensity a t longer processing times. This peak seems to be absent from some extracts, possibly due to loss of some of the low molecular weight material by evap0rati0n.l~At 320 "C, the extracts obtained after 90 min of treatment time and longer have practically the same molecular weight distribution. These results are summarized in Figure 3, in which the number-average and the weight-average molecular weights are given as a

0 MIN 20 MIN 60 MIN 90 MIN 120 MIN

300 MIN

X

300

700

1100 1500 1900 MOLECULAR WEIGHT

2300

Figure 2. Molecular weight distributions of the 320 "C extracts. YOLECUUR WEIGHT AVERAGES FOR THE 320 C EXTRACTS

1190

V'V'V'V8"

NUUBER AVERAGE YOL. WEIGHT WEIGHT AVERAGE YOL WEIGHT

I 0

60

120 180 TIME (MIN)

240

300

Figure 3. Number-average and weight-averagemolecular weights of the 320 "C extracts. The lines through the data are based on

the continuous reaction model.

function of time. These molecular weight averages increase during the first 90 min and remain constant thereafter. The molecular weight distribution data for the extracts obtained at 340 "C are presented in Figures 4 and 5. For extracts obtained in less than 1 h (Figure 41, the molecular weight distribution shifts towards higher molecular weights during the first 20 min and then remains practically constant until about 1 h of reaction time. The extracts obtained at reaction times longer than 1 h show a gradual evolution toward lower molecular weights. Since this shift occurs while the conversion remains practically constant, the most logical explanation is thermal degradation of the extracts at 340 "C, which most likely starts taking place from the first moments of treatment at this higher temperature. That its effect on the molecular weight distribution is felt only after processing times longer than 1 h is the result of two competing phenomena: recovery of increasing amounts of higher molecular weight material, as was the case at lower temperatures, and thermal degradation of the already recovered material, leading to a maximum in the average molecular weight of the extracts.



MOLECULAR WEIGHT AVERAGES FOR THE 340 C M R A C T S

0

__________ ____ ________ ....................

I

0 MIN 20 MIN 30 MIN 50 MIN 60 MIN

1210'

7g 0 NUYBER AVERAGE UOL WEIGHT

0 WEIGHT AVERAGE UOL WEIGHT

650 0

120

60

180

240

TIME (MIN)

300

700

1100

1500

1900

2300

MOLECULAR WEIGHT

Figure 4. Molecular weight distributions of the 340 "C extracts obtained in 1 h or less of treatment time.

Figure 6. Number-average and weight-average molecular weights

of the 340 O C extracts. The lines represent merely a smoothing of the data.

"

0.36 0

___ ___ _ __ _ ___ _

________ ....................

0

90 MlN 120MIN 182MIN 240 MIN

0

-0.29

6

lil

0

c0.22

i

51 0 . 1 4 83 8

0.07

0.00

0

0

..Q*L .I 1900 2300

" " " ~ " I " " ' ~ " ' I " " ~ " " I " ' ~ ' " ' ' I ~ " " "

300

700

1100 1500 MOLECULAR WEIGHT

Figure 5. Molecular weight distributions of the 340 "C extracts obtained in 1 h or more of treatment time.

This hypothesis is confirmed by the data of Figure 6, which gives the number-average and the weight-average molecular weights for the 340 "C extracts. The behavior is similar to the one observed by Yurum et a1.,12which they suggested results from disintegration of large aggregates held together by physical interactions. Miknis et a1.6 reported that the molecular weight of bitumen (the benzene-soluble material) obtained upon thermal decomposition of a Colorado oil shale passes through a maximum. The authors cited a hypothesis of bitumen being a mixture of tar and unconverted kerogen and suggested the change in the proportions of these constituents as an explanation for the change in molecular weight and composition observed for bitumen. The data of the present work show that the decrease in the molecular weight is due to thermal degradation of some components of the extracts. Moreover, the fact that this degradation depends strongly on

60


300

360

Figure 7. Conversion to the molecular weight fraction 518-563 at different temperatures.

temperature suggests its nature to be chemical rather than physical. Conversion to Narrow Molecular Weight Fractions. Previous work' has shown that there is a relationship between the molecular weight of organic compounds and the rate of their recovery as soluble species. To explore this relationship further, the same approach was used in the present work, namely determining the percent conversion, on a DBFS basis, for each of 21 narrow molecular weight fractions. The 320 "C data in the present study were similar to the 300 "C data of the earlier work in that the conversion to each fraction increased with treatment time and tended to an asymptotic value at long processing times. Typical data are presented in Figures 7-10, with the 300 "C data included for comparison. These data show no evidence of thermal degradation even for high molecular weight material at 320 "C just as was the case at 300 "C. Furthermore, the data for each molecular weight fraction were also adequately fitted by a first-order model in confirmation of our previous findings. The lines through the data are based on the first-order model. The dependence

212 Energy & Fuels, Vol. 3, No. 2, 1989 -----.e----.-o

0.39

Tahiri et al.

_.___ .-----.___

0.65'

0

, .'

*

.Q-,

0 '

f

-0.52'

t:

'\., &,,

:0

8

3 0 0 C RUNS

0 S20C RUNS

.".,,............. ...........'.. p

0.00 0

60


300

0

360

Figure 8. Conversion to the molecular weight fraction 1068-1158

at different temperatures.

60


300

360

Figure 10. Conversion to the molecular weight fraction >E88


0.3E

-0.29

t:

0

8

c 0 c0.22

2

5 2 s

Z0.14

a 0.07 0.00

6

0

60


300

360

320

720

1120

1520

1920

MOLECUUR WEIGHT

Figure 9. Conversion to the molecular weight fraction 1258-1358

Figure 11. First-order kinetic parameter as a function of molecular weight at 320 O C .

of the first-order rate parameter on molecular weight at 320 "C is shown in Figure 11. Except for the scatter observed in the low molecular weight region, there is a smooth dependence of the rate parameter on molecular weight. The continuous kinetics approach presented earlier for modeling the solubilization of the organic matter of this shale based on the 300 "C data' also works very well for the 320 "C data.13 An example of the results of that model for predicting the temporal evolution of the molecular weight distribution is shown in Figure 12. The lines through the average molecular weight data presented in Figure 3 are based on that model. The conversion to the narrow molecular weight fractions at 340 "C exhibited a behavior different from that observed at the lower temperatures. At this higher temperature, the conversion data to molecular weight fractions below 900 show a continuous increase with treatment time even af'ter periods of time as long as four hours. For fractions in the 900-1070 molecular weight range there is an initial increase and then a leveling off of the percent conversion at long processing times. The percent conversion to molecular

weight fractions above 1070 show an increase during the first hour or so and then a decrease a t longer times. Furthermore, for these fractions, the higher the molecular weight the higher the rate of decrease in their conversions. Typical examples of this behavior are shown in Figures 7-10. These results clearly show that the decrease in molecular weight observed for the 340 "C extracts at longer treatment times is due to a decomposition of the large molecular weight compounds to form lower molecular weight material. Note that for most of this time period, the overall conversion is constant at its asymptotic limit (Figure 1). The maximum in the conversion to high molecular weight fractions is due to a simultaneous generation of these compounds from kerogen and their thermal degradation. If each molecular weight fraction is designated by an index i, let P(i,t) be the percent conversion to fraction i at time t. For molecular weight fractions above 1070 one can write the rate of change of P(i,t) with time as




*/I

5.951 TEUP. 320 C TlUE 60 UIN

....................

A YW FRACTION 1258

- 1356

/

5.38'

EXPERIMENTAL COMPUTED

/

I

0 ........r.... 300 700

.

I\,

I

, , ,

1100 1500 MOLECULAR WEIGHT

/

/

/

I

/'

.,

1900

1.62

2300

Figure 12. Comparison of computed and experimental molecular weight distributions of the extracts obtained at 320 "C and 1h of treatment time.

The subscripts G and D refer to generation and degradation respectively. If it is assumed that the rate of generation for these heavy fractions is governed by the same first-order process observed a t lower temperatures and that the rate of degradation at a given moment is proportional to the amount of that particular fraction present in the system a t that instant (a first-order decomposition), the rates of generation and degradation will then be given by

= k:P(i,t)

1.67 1.70 1000 X l/T(K)

1.72

1.75

Figure 13. Arrhenius plot for the parameter ki for the three molecular weight fractions whose conversion data are shown in Figures 8 and 9.

0

U S E D ON THREE TEUPERATURES

3 5

0

e30.8'

s

0 0

E 6 g 22.2'

[ F],

1.65

0

A

(7) A'

In the above expressions, ki and k : are, respectively, the rate constants for generation and degradation of the compounds in the molecular weight fraction. P(i,t)designates the conversion to the molecular weight fraction, i, if compounds of such weight fraction were not subjected to thermal degradation. This hypothetical conversion is given by P"(i,t) = Pm(i)- [Pm(i)- POW]exp(-kit) (8) Summation of (6) and (7) yields dP(i,t) + k!,P(i,t)= ki[Pm(i)- PO(i)]exp(-kit) dt

(9)

where P(i,O) = Po(i),whose solution is given by

The development of (10) is simplistic since it was assumed that the generation of heavy fractions comes only from kerogen, with no allowance for generation from the degradation of compounds in fractions heavier than their own. Taking these processes into account would require some knowledge about the stoichiometry of the thermal degradation, which is not available. Because of lack of information in this area, no attempt has been made to

A A A

-."

I

325

641

957 1273 1589 MOLECULAR WEIGHT

1905

Figure 14. Activation energy as a function of molecular weight.

model the conversion to low molecular weight species whose percent production showed no decrease with treatment time. Equation 10 was fitted to the experimental data for the fraction with a molecular weight higher than 1070. Examples of the results of these fits are presented in Figures 8-10 where the lines through the 340 "C data are based on eq 10. The fit to the experimental data is satisfactory, which is not unexpected since four adjustable parameters are used. Whether these adjustable parameters carry any physical significance can be examined in terms of the temperature dependence of the kls, since these parameters have been determined at 300 and 320 "C.Figure 13 shows typical results for the familiar Arrhenius plot. In general the temperature dependence of the ki's was found13 to be adequately described by the Arrhenius equation over the temperature range examined. Plots of the Arrhenius parameters versus molecular weight are presented in Figures 14 and 15. Different symbols are used whether the determination was based on two or three temperatures. Examination of these data reveals that, depending on the molecular weight, basically

214

Energy & Fuels, Vol. 3, No. 2, 1989

Tahiri et al. Table VI. Atomic Ratios to Organic Carbon for Various Elements in the Extracts and Residues of the 320 OC ExDeriments

BASED ON THREE TEUPEPATURES

P 514'

3

A

7

0

I

A

641

957

1273

1589

1905

MOLECULAR WEIGHT

Figure 15. Frequency factor as a function of molecular weight.

three types of behavior are exhibited in terms of the values of the activation energy and frequency factor. The recovery of soluble material with a molecular weight less than 440 involves an activation energies of about 20 kcal/ (g mol) and a fairly high frequency factor of an order of magnitude in the 104-105 range. However, the scatter in the data in the low molecular weight range limits the confidence in these values. The recovery of soluble compounds with a molecular weight in the 440-1070 range involves an activation energy that is moderately increasing with molecular weight from about 8 to 1 2 kcal/(g mol) and a frequency factor below 315 min-l. These values suggest a diffusional mechanism as leading to the recovery of these compounds rather than chemical reactions. Finally, soluble compounds with a molecular weight higher than 1070 are recovered with an activation energy that increases significantly with molecular weight from a value of about 17 to about 46 kcal/(g mol). The frequency factors for these compounds also increase by several orders of magnitude with molecular weight, ranging from about 5 X lo4 to about 1015 m i d . These values are well within the range expected for chemical reactions. Mechanistic and Structural Considerations. In our earlier paper,' the view was presented that the kerogen of the Moroccan oil shale may consist of two components, a macromolecular gel and a substantial amount of individual molecules entrapped in that gel. It was further suggested that the release of the individual molecules upon heat plus solvent treatment may be an important mechanism in the recovery of substantial amounts of soluble organics at tempertures below those at which extensive pyrolysis occurs. The present data lend partial support to these hypotheses. As mentioned previously, the recovery of compounds with a relatively low molecular weight involved Arrhenius parameters that are so low as to suggest diffusional processes. Thus, it seems that these compounds are indigenous to the organic matter of this oil shale as such. The concept of "guest molecules" in the macromolecular structure of coal is currently an issue under debate.14-19 4) Given, P. H.; Mazrec, A.; Barton, W. A.; Lynch, L. J.; Gerstein, 1986, 65, 155-163. 5) Jurkiewicz, A.; Mazrec, A.; Piselewski, N. Fuel 1982,61,647-650. 6 ) Jurkiewicz, A.; Mazrec, A.; Piselewski, N. Fuel 1983, 62, 996-998. 7) Singleton, K. E.; Cooks, R. G.;Wood, K. V.; Rabinovich, A. Given, . Fuel 1987,66, 74-82. 8 ) Given, P. H.; Marzec, A. Fuel 1988, 67, 242-244. 9) Nishioka, M.; Larsen, J. W. Energy Fuels 1988, 2, 351-355.

. Fuel

1.33 1.32 1.29 1.32 1.25

320 "C Extracts 9.53 8.03 7.27 6.24 5.82

3.46 3.11 2.85 2.86 3.00

2.93 3.15 3.27 3.10 2.98

0 20 60 120 240

1.72 1.65 1.68 1.76 1.71

320 "C Residues 26.71 25.78 26.76 26.94 24.30

5.28 5.08 5.29 6.14 6.06

10.94 11.19 11.77 12.36 13.70

A

lA

325

0 20 60 120 300

That controversy seems to focus on what percentage of the organic matter of coal do physically trapped molecules represent rather than on their existence, which is widely accepted. Furthermore, when a good solvent is used, there is evidence that the extraction of the physically entrapped molecules in coal can be subject to kinetic limitations induced by an activated diffusion m e ~ h a n i s m . ' ~ The recovery of high molecular weight species involves high activation energies and frequency factors suggesting chemical rather than physical processes as the recovery mechanism. Even though the nature of these presumed chemical reactions is not known with certainty, it seems unlikely that they involve C-C bonds at the temperatures used. The data on elemental analyses for the 320 "C experiments showed a decrease in both oxygen and sulfur concentrations in the residues while the nitrogen concentration was practically constant, which suggests that if chemical bonds are broken, they might involve oxygen and/or sulfur functional groups. It is also not clear why the activation energy and frequency factor increase so dramatically with molecular weight for the heavy compounds where bond breaking might not be expected to be a function of molecular weight. The degradation of the larger molecules observed at 340 "C showed a strong dependence on temperature, indicating it is chemical in nature. Because of the decrease observed in the percentage of oxygen in the 340 OC extracts, it is likely that the thermal degradation involves oxygen linkages. Some interesting aspects of the elemental composition of this oil shale are shown in Table VI where the atomic ratios to organic carbon are presented for the 320 "C extracts and residues. Only non-carbonate oxygen was taken into account for the oxygen to organic carbon ratio for the extracts. The data of Table IV show the atomic ratios of various elements to be higher in the residues than in the extracts. These results are to be expected for sulfur and oxygen, which are associated with substantial amounts of nonorganic components of the shale, i.e., pyrite for sulfur and various mineral oxides for oxygen. However the amounts of nitrogen and hydrogen that would be associated with mineral matter are likely to be small. In this respect the data of Table VI suggest that the organic matter that has not been recovered in the toluene experiments contains substantial amounts of heteroelements, more than would be inferred from the elemental composition of the extracted organic matter, which is in accordance with conclusions reached by other worker^^-^ that the organic matter most refractory to recovery from oil shale is richer in heteroatoms, especially nitrogen. The significantly higher atomic hydrogen to carbon ratios of the residues compared to the extracts were somewhat


Kinetics of Kerogen Solubilization unexpected since it is widely believed that the organic matter that is refractory to thermal treatment is generally more aromatic. The observation about the atomic hydrogen to carbon ratio seems to suggest that the organic matter which was not recovered in these experiments involves saturated hydrocarbon structures that are either straight and/or cyclic. Conclusions From 30% to 44% of the insoluble organic carbon contained in an oil shale specimen from the Timahdit formation (Morocco) can be recovered as soluble material by thermal treatment in a toluene solution at moderate temperatures in the range from 300' to 340 OC. The highest yield achieved is in the range of Fisher Assay conversions, which involves thermal pyrolysis a t temperatures well above 500 OC. The elemental composition of the extracts was not dramatically affected by either time or temperature and was on the average as follows: C, 76%; H, 8%; 0,7%; N, 3%; S, 6 % . The percentage of oxygen in the extracts showed some decrease with treatment time due to a shift in composition as more extract was recovered and/or thermal degradation reactions occurred a t 340 OC. The elemental composition of some solid residues of the toluene treatment showed that the mineral carbonates of the shale did not undergo thermal decomposition under the conditions investigated. The fact that the carbonate minerals were not thermally decomposed seem to preclude the liberation of organic compounds that may be chemically bound to them as a major mechanism for the recovery of soluble organic matter in these experiments. The data indicate that the organic matter which remains refractory to toluene treatment contains a significant amount of heteroatoms, especially nitrogen. However, the hydrocarbon moieties of this refractory organic matter seem to be of a more saturated character than would be inferred from the extractable organic matter. The 'H NMR data showed that the extracted organic material has a predominantly aliphatic structure that was not substantially affected by either treatment time or temperture. GPC analyses data consistently showed the extracted organic material to have a molecular weight distribution extending from about 300 to about 2400. The molecular weight distribution of the extracted material was found to change with both time and temperature in a consistent manner. At the two lower temperatures, heavier species were recovered in increasing yields as the overall yield increased. At 340 "C the dynamics of the molecular weight were governed by two factors: recovery of more of the high molecular weight compounds and their thermal degradation to species of lower molecular weight. These competing factors led to a maximum in the average molecular weight of the extracts obtained at this temperature. From a kinetic standpoint, it was found that the data for conversion to soluble material can be fitted quite ad-

equately by using a simple first-order model whose kinetic parameter was well described by the Arrhenius law. An activation energy of 17 kcal/mol and a frequency factor of 31 360 min-' were determined. Further analyses of the data showed, however, that the actual mechanism is more complex and involves a set of individual first-order processes whose kinetic parameters depend on molecular weight. Activation energy computations showed that the recovery of compounds with a molecular weight less than 1070 amu involved activation energies in the neighborhood of 10 kcal/(g mol). Compounds with a higher molecular weight involved an activation energy that increased with molecular weight from 17 to about 46 kcal/mol. These results suggested a hypothetical model for kerogen in which compounds with a molecular weight lower than approximately 1100 were indigenous to oil shale as individual molecules that were trapped in a macromolecular gel and could not be extracted unless that gel was "relaxed" by heat and solvent induced swelling and cleavage of some thermally labile bridges. The recovery of relatively low molecular weight material seems to proceed as a result of a diffusional mechanism. The activation energies involved in the recovery of the high molecular weight material were sufficiently high to suggest the nature of the mechanism involves chemical reactions. These high molecular weight moieties were apparently solubilized as a result of the cleavage of thermally labile linkages involving oxygen and/or sulfur. Acknowledgment. The Fulbright fellowship for M.T. provided by the Moroccan-American Commission for Educational and Cultural Exchange and coordinated by Amideast is gratefully acknowledged. Our appreciation to ONAREP for providing the samples used in this study is also acknowledged. Funds for this research were also provided, in part, from the O.U. Energy Resources Institute and the Oklahoma Mining and Minerals Resources Research Institute. Nomenclature index for molecular weight fractions first-order kinetic parameter, min-' first-order kinetic parameter for the production of ki compounds in molecular weight fraction i k'i first-order kinetic parameter for the decomposition of compounds in molecular weight fraction i percent conversion to soluble material on a dry bitumen-free shale basis asymptotic percent conversion to soluble material percent conversion to soluble material at time 0 percent conversion to molecular weight fraction i at time t asymptotic percent conversion to molecular weight fraction i percent conversion to molecular weight fraction i at time 0 Registry No. Toluene, 108-88-3.

i K

Effect of temperature on the kinetics of kerogen ... - ACS Publications

Recommend Documents