Ind. Eng. Chem. Process Des. Dev. 1985,2 4 , 271-273
27 1
Effect of Reduced Pressure on Oil Shale Retorting. 2. Oil Yield Hyun S. Yangt and Hong Yong Sohn' Departments of Fuels Engineering and of Metallurgy and Metallurgical Engineering, University of Utah, Sa# Lake City, Utah 84 I 12
The effect of reduced pressure ranging from 0.05 to 0.86 atm on oil yield was determined as a function of heating rate, the flow rate and type of sweep gas, and the grade of oil shale. A lower pressure, higher heating rate, and higher flow rate of sweep gas produced a greater amount of oil. Retorting of Colorado oil shale at 0.05 atm with a heating rate of 12 K/min produced oil yield as high as 107 wt % of Fisher Assay (FA) with nitrogen and 112 wt % of FA with water vapor. The oil yield was substantially the same with nitrogen and carbon dioxide. The Michigan oil shale, containing a higher fraction of aromatic carbons than the Green River Formation shale, produced a higher percentage of FA oil yield under a reduced pressure.
Introduction A number of previous investigators (Bae, 1969; Wise et al., 1979; Noble et al., 1981; Burnham and Singleton, 1982) studied the effect of elevated pressure on oil yield. A higher pressure and lower heating rate produced a lower oil yield. Several factors which caused a lower oil yield at an elevated pressure were mentioned in previous studies. Bae (1969) noted that a high pressure caused slow transport of oil vapor from the sample matrix. Campbell et al. (1978) and Burnham et al. (1982) noted that oil degradation occurred mainly in the liquid phase or condensed phase, and not in the vapor phase. As the pressure increases, the oil fraction in the liquid phase increases; thus the degree of oil degradation increases. Burnham and Taylor (1979) noted that thermal cracking reactions are surface catalyzed and that the amount of cracking depends upon surface area of the spent shale. Another factor which lowers oil yield at a higher pressure is that the rate of cracking of oil vapor increases with increased pressure. Voge and Good (1949) reported an increase in the rate constant from 0.0026-0.0032 s-l at atmospheric pressure to 0.0078 s-l at 21 atm. Experimental Section A schematic diagram for oil shale retorting and its procedure have been reported elsewhere (Sohn and Yang, 1985). A series of superficial flow rates were used at different heating rates to determine the changes in oil yield under reduced pressures ranging from 0.05 to 0.86 atm. In addition, sweep gases of nitrogen, carbon dioxide, and water vapor and two different grades of oil shales obtained from the Anvil Points Mine in Colorado were used. Oil shales from Utah and Michigan were also studied. Results Effect of Reduced Pressure at Different Flow Rates. The superficial flow rate of sweep gas used in this study was measured with either a rotameter or a soapbubble test meter and corrected to 25 "C and 0.86 atm. The space velocity was calculated with the flow rate of sweep gas at 25 OC and 0.86 atm and theSree volume in the packed bed of shale particles. The free volume was determined to be 0.43 cm3/g of raw shale. For the determination of oil yield vs. superficial flow rate of sweep gas, experiments were carried out a t a heating rate of 12 K/min with 36.6 f 0.6 gal/ton of Colorado oil shale. Nitrogen was used as the sweep gas in this study. The oil
* Departments of Fuels Engineering and of Metallurgy and Metallurgical Engineering. Department of Fuels Engineering. 0196-4305/85/1124-0271$01.50/0
yield measured at atmospheric pressure under a heating rate of 12 K/min and nitrogen flow rate of 60 cm3/min was the same as the FA (Fischer Assay) yield. A 60 cm3/min was experimentally proved, as seen in Figure 1, to be sufficient to remove the produced oil quickly from the retort and low enough to efficiently condense it at the collecting system at 0.86 atm. It is seen in Figure 1that flow rate has a greater effect on oil yield at 0.86 atm than at 0.05 atm. Under a lower pressure, even a lower flow rate of sweep gas was shown to be quite sufficient to remove the oil produced. For instance, under 0.86 atm the oil yield increased until a superficial flow rate became about 60 cm3/min, and it leveled off thereafter. However, about 4 cm3/min flow rate was observed to be sufficient under 0.05 atm. Effect of Reduced Pressure at Different Heating Rates. In this study, a nitrogen flow rate of about 60 cm3/min at 25 OC and 0.86 atm was used. The sample which came from the Anvil Points Mine in Colorado had a grade of 36.6 f 0.6 gal/ton. Shown in Figure 2 is the oil yield as a function of heating rate at different pressures. A higher heating rate produced a higher oil yield at all the pressures studied. The oil yield was more sensitive to the heating rate at a higher pressure. The greater effect of heating rate at a higher pressure is considered to be due to the greater difference in residence time of oil vapor and mist produced during retorting at a higher pressure than at a lower pressure. Another reason for the higher oil yield at a higher heating rate is that in this case a greater amount of oil is produced at a high temperature, and thus a smaller amount of oil in the liquid phase is produced. As the oil fraction in the liquid phase decreases, the amount of oil recovery increases because of lower oil degradation, which mainly occurs in the liquid phase as mentioned above. Effect of Reduced Pressure for Different Sweep Gases. In this study, nitrogen, carbon dioxide, and water vapor were used for retorting under several reduced pressures. As discussed in the above section, an optimal superficial flow rate of 60 cm3/min corrected to 25 "C and 0.86 atm was used for retorting with nitrogen or carbon dioxide. For water vapor retorting about 0.1 g/min of water was used. A heating rate of 12 K/min and a 36.6 f 0.6 gal/ton of Colorado oil shale were used. As seen in Figure 3, oil yield was substantially the same with nitrogen or carbon dioxide in the pressure range used in this study. However, retorting with water vapor produced a higher oil yield than with nitrogen or carbon dioxide. Retorting with water vapor at 0.05 atm with a heating rate of 12 K/min produced oil yield as high as 112 wt % of FA. Retorting with nitrogen or carbon dioxide 0 1985 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 2, 1985
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Superficial Sweep Gas Flow Rate, Ctn3/min 1 5 10 50
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Figure 3. Effect of sweep gas on oil yield under reduced pressure.
Figure 1. Effect of space velocity on oil yield under various reduced
7
pressures (heating rate = 12 K/min).
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Figure 4. Effect of oil shale grade on oil yield under reduced
pressures (nitrogen flow rate of 60 cm3/min at 25 "C and 0.86 atm).
pressure (heating rate = 12 K/min, nitrogen flow rate = 60 cm3/min at 25 "C and 0.86 atm).
produced 107 wt % of FA at 0.05 atm. The oil yield at 0.86 atm with the same heating rate was 108 wt % of FA with water vapor, and 101 w t % of FA with nitrogen. Effect of Reduced Pressure for Different Grades of Oil Shales. Oil shales of two grades (36.6 f 0.6 and 27.0 f 0.5 gal/ton) obtained from the Anvil Points Mine in Colorado were retorted to determine whether the grade of oil shale has any effect on oil yield under reduced pressure. A heating rate of 12 K/min and a nitrogen flow rate of about 60 cm3/min corrected to 25 "C and 0.86 atm were used. As seen in Figure 4, the effect of pressure was similar for both samples. In both cases oil yield was approximately 107 wt % of FA at 0.05 atm. Resulting data indicate that the oil shale which have similar compositions of organic
matter produced similar changes in oil yield with pressure. It should be noted, however, that the grades tested were rather close to each other. Effect of Reduced Pressures for Oil Shales Obtained from Different Geographical Deposits. The three shales used in this study were obtained from the Green River Formation in Colorado and Utah and from the Antrim Basin in Michigan, near Alpena. A heating rate of 12 K/min at a nitrogen flow rate of about 60 cm3/min corrected to 25 "C and 0.86 atm was used. As seen in Figure 5, the change in oil yield from the Michigan shale was more sensitive to pressure than those from the Green River Formation shales. The oil yield from the Michigan shale was about 109 wt 70of FA at 0.05 atm while it was about 107 wt % of FA from the Green River
Figure 2. Effect of heating rate on oil yield under various reduced
Ind. Eng. Chem. Process Des. Dev., Vol. 24, No. 2, 1985 273
of heating rate and the flow rate of sweep gas on oil yield decreased with decreased pressure. Retorting with water vapor produced a higher oil yield than retorting with nitrogen or carbon dioxide at all the pressures studied. Under a lower pressure, even a lower flow rate of sweep gas was shown to be sufficient to remove the produced oil rapidly from the retort. The reasons for the higher oil yield under reduced pressure are as follows: (1)a smaller amount of oil in the liquid phase, which results in lower degradation of oil produced; (2) the recovery of high boiling aromatic compounds as oil by increasing the vapor fraction during retorting; (3) a shorter residence time of oil vapor, which causes a smaller amount of cracking of oil vapor into gases; and (4)a smaller rate constant for the cracking of hydrocarbons in oil produced during retorting. 0.037
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Michigan
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Figure 5. Variation of oil yield under various reduced pressures for the Eastern and Western oil shales (heating rate = 12 K/min, nitrogen flow rate = 60 cm3/min at 25 O C and 0.86 atm).
Formation shales at the same pressure. The effect of pressure was substantially the same in the Green River oil shales of Colorado and Utah. I t is known that a higher fraction of aromatic carbons is contained in the Eastern shale than in the Western shale. A high boiling aromatic fraction produces a greater change in oil yield at reduced pressure by increasing the amount of oil in the vapor phase and thus lowering the oil degradation. A comparison of the effect of heating rate on oil yield from the Western and the Eastern oil shales was made by Burnham et al. (1982). They observed that the effect of heating rate at atmospheric pressure on oil yield was much greater for the Eastern oil shale than for the Western oil shale. Conclusions A lower pressure, higher heating rate, and higher flow rate of sweep gas produced a higher oil yield. The effects
Acknowledgment The authors are thankful to Dr. I. C. Lee for many helpful discussions during the course of this work and to Dr. D. M. Bodily for allowing us to use many valuable items of equipment in carrying out this work. Registry No. Nitrogen, 7727-37-9; carbon dioxide, 124-38-9; water, 7732-18-5.
Literature Cited Bae, J. H. SOC.Pet. Eng. J . 1989, 9(3), 287. Burnham, A. K.; Taylor, J. R. Lawrence Livermore National Laboratory, Rept. UCID-18284, Livermore, CA. 1979. Burnham, A. K.; Richardson, J. H.; Coburn. T. T. Lawrence Lhrermore Nationai Laboratory, Rept. UCRL-87587, Livermore, CA, 1982. Burnham, A. K.; Singleton, M. F. Lawrence Livermore National Laboratory, Rept. UCRL-88127, Livermore, CA, 1982. Campbell, J. H.; Koskinas, G. J.; Stout, N. D.; Coburn, T. T. In Situ 1978, 2(1h 1. Noble, R. D.; Harris, H. G.; Tucker, W. F. Fue/ 1981, 60, 561. Sohn, H. Y.; Yang, H. S. Ind. Eng. Chem. Process Des. Dev. 1985, preceding article (part I)in this issue. Voge, H. H.; Oood, G. M. J . Am. Chem. SOC.1949, 7 1 , 593. Wise, R. L.; Miller, R. C.; George, J. H. Am. Chem. SOC. J., Div. Fuel. Chem. Prepr. 1979, 21(8), 87.
Received for review August 1, 1983 Revised manuscript received April 19, 1984 Accepted May 21, 1984 This work was supported in part by a Research Grant from the University of Utah Research Committee and the University of Utah College of Mines and Mineral Industries Mineral Leasing
Fund.
