CATALYTIC HYDROCRACKING Hydrocracking of a Low Temperature Coal T a r S.
A .
Q A D E R
A N D
G.
R.
HILL
Department of Mineral Engineering, University of Utah, Salt Lake City, Utah 84212 The hydrocracking of a low temperature coal tar in a batch autoclave over a dual-functional catalyst containing sulfides of nickel and tungsten supported on silica-alumina yielded 75% gasoline of premium grade. The dual-functional catalyst gave better product distribution with respect to gasoline, while the refining catalyst containing sulfides of tungsten and nickel on alumina gave a better product distribution with respect to the yield and quality of diesel oil. The dual-functional catalyst exhibited higher cracking, isomerization, and hydrogenation activities compared to the conventional cracking and refining catalysts.
HYDROCRACKING has been investigated
in recent years as a potential method for upgrading coal-derived liquid fuels. The earlier work was mainly done in connection with the development of coal hydrogenation processes (Gordon, 1935, 1946, 1947; Kronig, 1949; Pier, 1949), where a two-stage process was used in which coal was liquefied in the first stage, and the resulting heavy oil was hydrocracked in a subsequent stage to gasoline. Almost all the work reported so far was done on either thermal or catalytic hydrocracking of coal liquids over refining or hydrogenation catalysts (Alpert et al., 1964; Carpenter et al., 1963; Central Air Documents Office, 1951; Rutkowski, 1965). In recent years, dual-functional catalysts containing both hydrogenation and cracking activities were developed for the hydrocracking of petroleum fractions (Larson et al., 1962; Myers et al., 1962). These catalysts contain hydrogenation and cracking components and yield improved product distributions (Archibald et al., 1960; Flinn et al., 1960). In the present communication, the results of the hydrocracking of a low temperature coal tar in a batch autoclave over a dualfunctional catalyst are reported. Experimental
Materials. T a r (Table I) prepared from a high volatile bituminous coal from Utah by carbonization a t 550" C. in a laboratory oven was used as the feed material. The dual-functional hydrocracking catalyst contained 6% nickel and 19% tungsten, both as sulfides, supported on silica-alumina and had a surface area of 212 sq. meters per gram and a size of -200-mesh. The conventional cracking catalyst contained 84% silica and 16% alumina, with a surface area of 200 sq. meters per gram and -200mesh size. The refining catalyst contained 6% nickel sulfide and 19'2 tungsten sulfide, supported on alumina with a surface area of 210 sq. meters per gram and -200-mesh size. The catalysts were commercial products. Equipment. A 1-liter high pressure autoclave provided with a magnetic drive stirrer of 1800 r.p.m., pressureand temperature-control devices, liquid- and gas-sampling lines and water-quenching system (Figure l ) , and hydrogen cylinders containing pure hydrogen with a maximum pressure of 2300 p.s.i. were used. 450
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
Hydrocracking Procedure. In each experiment 100 cc. of tar and 15 grams of the catalyst were used. The equipment was evacuated to remove most of the air, filled with hydrogen, and heated to the desired temperature. The temperature rose to 300" C. in 20 minutes and 500" C. in 26 minutes. The reaction time was taken from the start of heating the equipment. When the reaction temperature was reached, the hot pressure was adjusted to the experimental value and maintained constant throughout, except in experiments conducted a t pressures higher than 2000 p.s.i., where there was a reduction in pressure of about 50 to 100 p.s.i. during the experiment. Experiments were carried out for different times a t different reaction temperatures (Figure 2): 10 hours a t 350" and 375"C., 8 hours a t 400" and 425"C., 6 hours a t 450" and 475"C., and 5 hours a t 500°C. However, the experiments designed for catalyst comparison (Figures 10 to 12) were all carried out for 10 hours. After the experiments, the reaction mixtures were quenched t o atmospheric temperature in about 10 minutes. Four gas samples were taken out during each experiment for analysis. After the autoclave was cooled down to the atmospheric temperature, the pressure was released and it was opened. The product was transferred to a beaker and filtered t o remove the catalyst, and the water separated. The mechanical losses were found to be less than 1%. The yield of the product was taken as looL> and 100 Table I. Properties of Low Temperature Tar Sp. gr., 250C. T a r acids, vol. 7~ Tar bases, vol. 5; Sulfur, wt. YC Nitrogen, wt. ti Distillation I.B.P., C. u p t o 200" C., vol. L'C 50LZ distillate, C. Pitch point, ' C. Pitch, vol. ?% Analysis of neutral tar up to 360" C., vol. 5 Saturates Olefins Aromatics
0.9894 27.0 2.0 0.8456 0.6438 89 15 283 360 26.0 28.0 15.0 55.0
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Figure 1. Assembly of equipment 1. Heating jacket
10. Stirrer controller
2. Thermocouple pocket
11. Vacuum pump
3. Magnetic drive assembly for stirrer
12. Hydrogen tank
4. Cooling coil for quenching
13. Ammeter
5 . Liquid-sampling line
14. Variacs
6. Gas-sampling line 7. Flowmeter
15. Relay 16. Temperature controller 17. Potentiometer connected to thermocouples
8. Pressure gage 9. Electric motor
minus the volume of the total oil product was taken as per cent conversion to gas. The total oil product was distilled and separated into fractions boiling up to 200°, 200" to 360"C., and residue. The neutral fraction boiling up to 200°C. was designated as gasoline and the one boiling from 200" to 360" C. as diesel oil. Coke Determination. In the experiments designed for coke determination, 20C grams of tar and 30 grams of the dual-functional catalyst were taken and the reaction was carried out a t 475' C. and 1500- to 3000-p.s.i. pressure for 6 hours. At the end of the reaction period, the product was cooled and taken from the autoclave. The catalyst was separated by filtration, washed with benzene, dried a t 110°C. for 2 hours, and re-used in the reaction. Each batch of catalyst was used for five experiments for hydrocracking 1000 cc. of tar a t each hydrogen pressure. After the fifth experiment, the catalyst was separated, washed with benzene, and dried. Fifteen grams of the dried catalyst was packed in a glass tube of 0.5-inch internal diameter, 12-inch length, heated to 600°C. by a tubular furnace. A stream of air was passed through the tube a t the rate of about 20 cc. per minute and the effluent was passed through a furnace containing cupric oxide a t 700" C. to oxidize carbon monoxide to carbon dioxide, a tower containing Drierite to adsorb any moisture, and a tower of Ascarite to adsorb carbon dioxide. The carbon on the catalyst was calculated from the weight of carbon dioxide and reported as coke. The catalyst deposit was not strictly carbon, but coke containing both carbon and hydrogen. The coke values reported actually refer to the carbon content of the coke, the former amounting to about 95% of the latter, the remaining being hydrogen.
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2
3
4
5
6
7
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2. Effect of reaction time on conversion Pressure, 1500 p.s.i.
Product Analysis. Sulfur was determined by the bomb method and nitrogen by the C-H--N chromatographic analyzer, F. M. Model 185. Tar acids and bases were estimated by extraction with 10% sodium hydroxide and 20% sulfuric acid, respectively. Hydrocarbon-type analysis was done by the fluorescent-indicator-adsorption method (ASTM, D 1319-6iT). For naphthene and isoparaffin estimation, the saturated portion of each fraction was first separated from the mixture by sulfonation with a mixture of 70% concentrated sulfuric acid and 3 0 5 phosphorus pentoxide (ASTM, D 1019-62). The naphthenes were estimated by the refractivity intercept method (ASTM, D 1840-64). The normal paraffin content was determined by adsorption over 5-A molecular sieves in a glass column of 0.5-inch diameter and 1.5-foot length. The isoparaffins were obtained by difference. The diesel index was calculated from API gravity and aniline point. The gas analysis was done by gas chromatography in the F. M. Model 720 dual-column programmed temperature gas chromatograph and by mass spectrometry. Results and Discussion
The hydrogen consumption varied between 2 and 3;' and water formation was about 1 to 3% by weight of tar. Since hydrogen consumption was approximately equal to the water formed in the reaction, the yield of the total product, excluding water, H?S, and NH?, was taken as 100% by volume of the feed tar. Since a part of the catalyst might be deactivated by the impurities in tar, a larger excess of the catalyst (1541) than the optimum quantity (about 10%) required (Figure 3) was used in these experiments to ensure enough catalyst throughout the course of the reaction. VOL. 8 NO. 4 OCTOBER 1969
451
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CATALYST
100
125
150
WT. % OF T A R
Figure 3. Effect of catalyst concentration on gasoline yield Temperature, 450" C.; pressure, 1500 ps.i.
Influence of Temperature. Temperature had a marked effect on product distribution (Figures 4 and 5). The yields of gasoline and gas increased with temperature, with a corresponding decrease in diesel oil and residue. The gasoline and gas were mainly formed by cracking of the diesel oil. The quality of the diesel oil decreased with the reaction temperature (Figure 4). Cracking of the diesel oil mainly involves the splitting of saturated materials, leaving behind the more refractory aromatics having low diesel indices. The rate of removal of sulfur, tar acids, and nitrogen was high up to 400"C., as was the formation of total neutral product and the aromatics of the product (Figure 5). The results indicated that the increase in the total neutral product was mainly due to the hydrogenation of sulfur compounds, tar acids, and nitrogen compounds with the formation of the corresponding aromatic hydrocarbons, leading to an increase in the total aromatics of the product. Desulfurization, tar acid hydrogenation, and denitrogenation were 100%
a t 450", 475", and 500°C., respectively, and, hence, the reactions were not subject to any thermodynamic limitations under the experimental conditions employed. The product distribution a t different levels of gasoline and diesel oil formation is shown in Figure 6. High gasoline conversions were associated with higher yields of gas, while the reverse was true in the case of diesel oil conversion. Higher yields of gasoline were also associated with better quality product. Isomerization in butanes increased with reaction temperature (Figure 4) and the gasoline contained high proportions of isoparaffins a t higher conversions (Figure 6). Catalytic hydrocracking produces more branched hydrocarbons than can be predicted by thermodynamic equilibrium, because of rapid cracking followed by equally rapid isomerization of the cracked fragments. The extent of isomerization depends upon the cracking reactions and the structure of the parent molecules. Flinn et al. (1960) obtained iso-normal ratios in butanes of 3.6, 3.0, 1.4, and 0.6 during the catalytic hydrocracking of cetane, Decalin, Tetralin, and n-butylbenzene, respectively. The ratios calculated from the hydrocracking of saturates were higher than those from aromatics. The hydrocracking of low temperature tar involves the cracking of a mixture of paraffins, naphthenes, olefins, alkyl aromatics, and hydroaromatics in addition to heterocyclic compounds and, hence, the iso-normal ratios in butanes represent the average values of the ratios that could be obtained with each type of compound separately. Influence of Pressure. The gasoline yield increased a t different rates with pressure (Figure 7). The rate of gasoline formation was high in the pressure range 1000 to 1500 p s i . , slowing down in the range 1500 to 2500 p s i . , and increasing again a t higher pressures. The aromatic content of the gasoline increased up to a pressure of
r 1 NEUTRAL LIQUID PRODUC' ( U P T C 3 6 0 'Cc)
70
2 AROMATICS
1.GASOLINE 2.DlESEL OIL 3. R E S I D U E 4.GAS
SULFUR TAR A C I D S hITROGEN
Y O t 350
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1
400 450 TEMPERATURE, " C
500
Figure 4. Effect of temperature on product distribution Pressure, 1500 p.s.i.
452
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
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Figure 5 . Effect of temperature on product distribution Pressure, 1500 p.s.i.
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2500 p s i , but decreased rapidly a t higher pressures. On the other hand, the naphthenic content of the gasoline increased slowly up to a pressure of 2500 p.s.i. and rapidly a t higher pressures (Figure 8). The rapid increase in the gasoline yield in the pressure range 1000 to 1500 p.s.i. might be due to the suppression of coke-forming reactions due to increase of pressure. In the range 1500 t o 2500 p.s.i., partial hydrogenation of aromatics to the corresponding hydroaromatics might be taking place with the subsequent cracking of the hydroaromatics formed. Above 2500-p.s.i. pressure, complete hydrogenation of aromatics to the corresponding naphthenes might occur, followed by subsequent cracking of the naphthenes, thereby effecting a rapid increase in gasoline yield and its naphthenic content. The yield and quality of diesel oil increased with pressure. Diesel oil of 57 diesel index was obtained a t 375°C. in a yield of 60‘70 (Figure 7). Desulfurization and tar acid hydrogenation increased linearly with pressure, while denitrogenation was slow up to a pressure of 2000 p.s.i. but increased rapidly a t higher pressures (Figure 9). The yield of neutral liquid product increased with pressure, while the aromatics in the product increased up to 2000-p.s.i. pressure but decreased rapidly a t higher pressures, probably because of their conversion to naphthenes a t higher pressures.
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Figure 6. Yields and quality of products at differeni levels of conversion
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Figure 8. Effect of pressure on yield and composition of gasoline 1. 2.
3.
Gasoline
Naphthenes
Aromatics
VOL. 8
N O . 4 OCTOBER 1 9 6 9
453
1 NEUTRAL LIQUID PRODUCT ( U P T O 360OC) 2 AROMATICS
1 SULFUR 2 TAR ACIDS 3 NITROGEN
30
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9. Effect
2000 PRE SSJR E, psi
3000
of pressure on product distribution Temperature, 450" C.
Catalyst Comparison. The product distribution obtained with the dual-functional hydrocracking catalyst (WSZ-NiSSi02-A1203) was compared with those obtained with the conventional cracking (SiOZ-Al2O3)and refining (WS2-NiSA1203)catalysts (Figures 10 to 12). The dual-functional catalyst exhibited higher cracking activity than the cracking catalyst, which in turn had higher activity than the refining catalyst, as indicated by the yields of gasoline and gas (Figure 10). Higher yields of better quality diesel oil were obtained with the refining catalyst. The higher cracking activity of the dual-functional catalyst is due to the presence of the hydrogenation component which minimizes coke formation and accumulation of other impurities on the active sites of the cracking component which thus maintains high activity. The cracking took place with the refining catalyst and was purely thermal and, hence, the yields of gasoline and gas were comparatively low. The refining catalyst exhibited higher activity for desulfurization and tar acid hydrogenation, while the dualfunctional catalyst showed maximum activity for desulfurization and denitrogenation (Figure 11). The curves in Figure 11 indicate that desulfurization and tar acid hydrogenation were more influenced by the hydrogenation component than the cracking component of the catalyst, while denitrogenation was affected by the combined activity of both the components.
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Figure 10. Effect of temperature on product distribution
Figure 11. Effect of temperature on product distribution
Pressure, 1500 p.s.i.
Pressure, 1500 p.s.i.
454
I & E C PROCESS DESIGN A N D DEVELOPMENT
T h e dual-functional catalyst exhibited higher isomerization activity than the cracking catalyst, which in turn had higher activity than the refining catalyst, as shown by the iso-normal ratios in butanes (Figure 10) and the isoparaffinic content of the gasoline (Figure 12). The results indicated that isomerization takes place mainly on the acidic sites of the cracking catalyst and is always followed by cracking. The incorporation of hydrogenation activity in the cracking catalyst prevents the establishment of equilibrium between iso- and normal compounds and, therefore, leads to the formation of a larger excess of branched isomers than can be predicted by thermodynamic equilibrium. This is due to the rapid saturation of the branched olefinic fragments formed during cracking before the latter readsorb to isomerize toward equilibrium. The dual-functional catalyst was more effective in the production of gasoline from tar, while the refining catalyst yielded better product distribution with respect to the production of diesel oil. The composition of gasoline (Figure 12) indicated that the dual-functional catalyst produces better quality produce. A maximum yield of 7 9 7 of gasoline was obtained from tar over the dualfunctional catalyst a t 500" C. and 3000-p.s.i. pressure, but the highest quality product containing 53% aromatics and 16% isoparaffins was obtained a t 500°C. and 2500-p.s.i. pressure in a yield of 75%, which can compare well with the premium grade gasoline from petroleum (Table 11). The material balance obtained with the dual-functional catalyst is given in Table 111. Coke Formation on Catalyst. The results of coke deposition on the catalyst during hydrocracking of low temperature tar are shown in Figure 13. With 30 grams of the catalyst, the deposit was found to be about 1% by weight for processing a feed of 1 liter a t 1500-p.s.i. pressure and 475"C., but was reduced to about 0 . 9 7 when the hydrogen pressure was raised from 1500 to 3000 p.s.i.
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Table II. Comparison of Gasoline from Tar and Petroleum Gasoline 1from tar a t 500" C. and 3000 p.s.i. Gasoline 2 from tar a t 5OOnC. and 2500 p.s.i.
Gasoline
from Petroleum
Yield, vol. % Composition of gasoline, vol. :5 Aromatics Saphthenes Olefins Isoparaffins n-Paraffins
Gasoline
Gasoline
1
2
79.0
75.0
27.0 35.0 2.0 21.0 15.0
53.0 17.0 2.0 16.0 12.0
Regular Premium grade
grade
36.0 17.0 4.0 28.0 15.0
42.0 10.0 6.0 30.0 12.0
Table Ill. Material Balance Feed. T a r oil 99 grams, hydrogen 2.8 grams
Products
Yields, G.
Gasoline Diesel oil Residual oil, +360" C. Gas Water Hydrogen sulfide Ammonia
54.25 14.25 11.50 0.55 2.40 0.50 0.70
05 1500
2000
2500
3000
PRESSURE, f?S.1
Figure 13. Coke deposition on the catalyst Temperature, 475' C.
literature Cited 40
2
IO
c
45
t
40 350
400
450
500
TEMPERATURE, ' C
Figure 12. Effect of temperature on gasoline composition Pressure, 1500 p s I 1 WSz-NIS-SIO - A l l 0 1
2 3
WSJ-NIS-AI,~ S102-AI201
Alpert, S. P., Johanson, E. S., Schuman, S. C., Chem. Eng. Progr. 60, No. 6, 35 (1964). Archibald, R . C., Greensfelder, B. S., Holzman, G., Rowf, D. H., Ind. Eng. Chem. 52, 745 (1960). Carpenter, H. C., Cottingham, P. L.,Frost, C. M., Fowkes, W . W., Bur. Mines Rept. Invest. 6237 (1963). Flinn, R. A., Larson, 0. A., Beuther, H., Ind. Eng. Chem. 52, 152 (1960). Gordon, K., J . Inst. Fuel 9, 69 (1935); 20, 42 (1946): 21, 53 (1947). Kronig, W., Ind. Eng. Chem. 41, 870 (1949). Larson, 0. A,, Maciver, D. S., Tobin, H. H., Flinn, R. A., IND.ENG.CHEM.PROC. RES. DEVELOP. I , 300 (1962). Myers, C. G., Garwood, W. E., Rope, B. W., Wadlinger, R . L.,Hawthorne, W. P., J . Chem. Eng. Data 7, 257 (1962). Pier, M., 2. Elektrochem. 53, Xo. 5, 291 (1949). Rutkowski, K., Zest. Nauk. Politech. Wrol. Chem. 12, 3 (1965). Chemical Section, Central Air Documents Office, Dayton, Ohio, "TTH Process, High Pressure Hydrogenation a t Ludwigshafen-Heidelberg," Vol. 4B, 1951. RECEIVED for review July 1, 1968 ACCEPTED May 24, 1969 VOL. 8 NO. 4 OCTOBER 1 9 6 9
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