Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 622-627
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Storage Stability of Synfuels from Oil Shale. 3. Studies with Actual Shale-Derived Middle Distillates John W. Frankenfeld’ and Wllllam F. Taylor Exxon Research and Engineering Company, Products Research Division, Linden, New Jersey 07036
Dennis W. Brlnkman Bartlesville Energy Technology Center, U.S. Department of Energy, Bartlesville, Oklahoma 74003
The storage stability of various middle distillate fuels derived from oil shale was investigated by means of an accelerated storage stability test. A variety of liquids were studied including crude shale oils boiling in the middle distillate range, partially upgraded shale oils, and severely refined fuels. Large amounts of sediment were obtained from liquids with high heteroatom content. However, no direct correlation between nitrogen, sulfur, no oxygen levels and sediment level was observed. The results of these studies are consistent with those from previous work with model fuel systems. The sediments produced by different liquids differed in heteroatom content and other characteristics. The nitrogen level of the original liquid was only one factor determining the amounts and types of sediments produced. Thus, studies with model compounds and with actual shale-derived liquids indicate that the total nitrogen content of a fuel per se is not a general predictor of fuel storage stability.
Introduction The current high costs and projected future shortages of petroleum-derived fuels have prompted the study of fuels derived from alternate sources, especially oil shale (so-called “synfuels”). Such materials differ in significant ways from their petroleum counterparts. Among these differences is the relatively high nitrogen content of synfuels (Frankenfeld and Taylor, 1980, Taylor and Hall, 1975; Drushel, 1969). This leads to storage stability problems manifested by the formation of highly insoluble nitrogenous sediments (Frankenfeld and Taylor, 1982; Thompson et al., 1951; Mapstone, 1949). Since little was known about such sediment formation, a study was undertaken to determine the causes and assess its probable impact on shale processing and product quality. Previous papers in this series (parts 1and 2, Frankenfeld et al., 1983a,b)presented the results of studies in model fuel systems. In part 1 the general features of the reaction (effects of nitrogen concentration, temperature, dissolved oxygen content, moisture, and light) were discussed. Part 2 was concerned with effects of chemical structure of the nitrogen compound and interactive effects. This paper discusses studies with actual shale middle distillates. The results are compared with those obtained in model fuel systems. Experimental Section Test Fuels. A variety of fuels derived from various oil shales and boiling in the middle distillate range were employed. These possessed a wide range of physical and chemical properties depending upon source, retorting techniques, and type and severity of upgrading. Inspections on the 13 fuels studied most extensively are given in Table I. Fuels A-J were prepared by distilling and hydrotreating a crude shale oil obtained from the Occidental Oil shale Company’s in situ process (Frankenfeld and Taylor, 1980). Fuels K and L were produced from Paraho shale by Sohio (Winward and Burdett, 1979) and fuel M was produced from Paraho shale oil by distillation followed by hydrotreating (Kalfadelis, 1976). Test Compounds. Two pure compounds were employed to “spike” selected fuels in order to study interactive effects. These were 2,5-dimethylpyrrole (DMP), an es-
pecially reactive nitrogen compound (Frankenfeld et al., 1983a, 1983b) and thiophenol, a typical aromatic thiol which had been studied previously (Frankenfeld et al., 198313). These were of the highest quality obtainable commercially. The DMP was redistilled prior to use and stored in the dark under an inert atmosphere. Accelerated Storage Stability Tests. Details of the test procedure were presented in part 1 of this series (Frankenfeld et al., 1983a). The tests were run at 43.3 “C (110 OF). Thirteen weeks at this temperature are considered equivalent to about one-year’s storage under ambient conditions (White, 1973). The results are presented as mg of sediment/100 cm3 of test fuel. Only total sediment levels (sum of “insoluble plus adherent”; Frankenfeld et al., 1983a) are given. Existent gum was measured by use of ASTM test D381. Results Stability of Various Shale Middle Distillates. The results of accelerated storage tests for crude and partially refined shale middle distillates are shown in Table 11. These results show that, although fuels with high nitrogen content generally give large amounts of sediment, there is no direct relationship between total nitrogen and sediment levels. Thus, fuel K afforded only one-tenth of the sediment produced by fuel C even though the two had the same total nitrogen content. Fuels C and K were obtained from different sources by different processing schemes. Some typical plots of sediment formation vs. storage time are shown in Figure 1. Linear plots, analogous to those in Figure 1, were obtained with most of the crude and moderate refined middle distillates. They are quite similar to those obtained with model nitrogen compounds of the pyrrole and indole type (Frankenfeld et al., 1983a; Frankenfeld and Taylor, 1982). These plots remain linear with time for extended storage periods (in excess of 100 days at 43.3 “C; Frankenfeld and Taylor, 1981, 1982). The results of accelerated storage stability tests for severely refined shale middle distillate are shown in Table 111. Existent gum tests were also carried out on fuels whose boiling range was low enough for the test to be valid. All of these fuels were quite stable for storage periods up to 56 days (equivalent to eight months at ambient tem-
0196-4321/83/1222-0622$01.50/00 1983 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983
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420
E
360 330
2
J
8 T
300 270
210
0
2 ?
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160
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150
-
60
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, 6
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42
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54
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78
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F-r-T-.r-r-o 90
96
F
1
102 108 114 120
3 STORAGE TIME IDAYSl
Figure 1. Sediment formation with various shale oil middle distillates (fuels D, E, and F) stored at 43.3 "C in the dark. U
c
e
3
4 3AY
4
3
3
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4
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Figure 2. Sediment formation with some severely hydrotreated shale middle distillates (fuels F, G , H, and I) stored at 43.3 "C in the dark.
perature). At longer time periods, however, sediment was observed in most of these fuels. This increased at an accelerated rate after the preliminary period was passed. Plots of sediment formation vs. storage time for these fuels are given in Figure 2. These show a linear response once the apparent initial induction period was exceeded. Existent gum levels were fairly high for these fuels after 28 days' storage. Once again no direct relationship between nitrogen or sulfur contents and sediment or gum levels was observed. Fuel M, a highly refined, shale-derived jet fuel, showed no tendency toward sediment formation when stored in the dark, although some soluble (existent) gum buildup was observed. However, a sample stored in the presence of sunlight exhibited a steady darkening and produced appreciable sediment within four months. A petroleumderived jet fuel of similar type gave no evidence of color or sediment under the same storage conditions (Frankenfeld and Taylor, 1982). Interactive Effects. The shale liquids reacted in varying ways when spiked with small amounts of 2,5-dimethylpyrrole (DMP). In some cases a strong positive interaction (more sediment than expected from the additive effects of the two fuels; Frankenfeld et al. 1982b) resulted, while in others a negative interaction was observed (less sediment than expected). These results are given in Table IV. It is significant that, in most instances (fuel D is the one exception), strong positive interactions were encountered between DMP and the shale liquids with high nitrogen content (B, C, and E), while strong negative
624 Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 4, 1983 Table 11. Sediment Formation in Crude and Partially Refined Shale Middle Distillates during Storage at 43.3 “C wt, PPln of N
fuel a
12000 12000 3600 10000 4000 3600
A
B C D E K
total sediment, mg/100 cm3 after
wt, PPm of s
28 days
6200 5600 2300 3000 103 0.6
See Table I for fuel characteristics.
60 days
___
70 days -__ 50 6 114 -__
411
_.. __.
29 7 46.9 79.6 39.1 4.3
191 74.1
84 days
___ ___
822 155 4 09 198 17.2
-__ 129
_._
10.5
-
__.
560
___
-_.
120 days
Average of three or more replicates.
Table 111. Sediment Formation in Refined Shale Middle Distillates during Storage a t 4 3 . 3 “C existent gum, mg/100 cm3, after 28 days
total sediment, mg/100 cm3 fuel a
F G H I J L M Md
N, wt, ppm 420 340 360 480 240 15
