Energy & Fuels 1994,8, 99-104
99
Iron-Based Catalysts for Coal/Waste Oil Coprocessing H. G. Sanjay, Arthur R. Tarrer,* and Chad Marks Department of Chemical Engineering, Auburn University, Auburn, Alabama 36849-5127 Received July 12, 1993. Revised Manuscript Received October 18, 1993'
Coprocessing coal with waste oil can achieve the dual purpose of recycling waste oil and liquefying coal economically. Waste oil is primarily paraffinic and is a poor hydrogen donor solvent but contains dispersant additives which could help improve dispersion of the coal particles and the catalysts during liquefaction. The initial coprocessing studies were conducted using 10% coal in tubing bomb reactors and a jet-loop reactor system developed a t Auburn University. Coal conversions in excess Of 85 % were obtained during coprocessing with selectivity of over 80% to oils. The use of iron-based catalyst precursors and a traditional hydrogen donor solvent such as tetralin with waste oil did not have a significant effect on conversion and selectivity during coprocessing. However, the sulfur removal and the ash removal from the waste oil increased. This study indicates that coprocessing coal with waste oil is beneficial, and an extensive study is under way a t present.
Introduction Coprocessing coal with waste materials such as used tires, waste plastic, and waste oil, all of which have expensive disposal costs, has the potential to improve the economics of coal liquefaction. There are very few reported investigations of using waste materials for coprocessing to liquefy coal.lf2 The Department of Chemical Engineering at Auburn University operates a pilot plant for reprocessing waste oil; this research facility recycles 2.5-3 million gallons of waste oil for use as fuel every year. Almost 1.2 billion gallons of waste oil are generated in the United States each year, posing an environmental hazard due to its metal bearing compounds and high sulfur content. The used oil must be re-refined and hydrotreated before use as a fuel or as a lube base stock. Reactions during the hydrotreatment of used oil include hydrodesulfurization (HDS), hydrodemetallation (HDM), and hydrodeoxygenation (HDO). The undissolved coal could act as a trap for the metals removed from the oil during coprocessing,while the sulfur present in the oil could serve to produce the sulfided catalyst needed for liquefaction. The overall objective of this work was to evaluate the beneficial effects of coprocessing coal with waste oil using iron-based catalysts. Waste oil is primarily paraffinic and is a poor hydrogen donor solvent. It does, however, contain surfactants which could be advantageous to liquefaction. Additives found in the waste oil, such as detergent/dispersant additives, oxidation inhibitors, etc., are mostly organic sulfur compounds. The additives present in waste oils include sulfonates, sulfides, dithiophosphates, etc3 These additives could help effectively disperse the coal and catalyst precursors throughout the coal/oil slurry during coprocessing. In addition, these additives can serve as sulfur sourcesto convert the catalyst precursors to the more active form. The unconverted coal could serve to trap the metals removed from the oil. The removal of metallic impurities Abstract published in Advance ACS Abstracts, November 15,1993. (1) Sanjay, H. G.; Tarrer, A. R.; Marks, C. Prepr. Pap.-Am. Chem. Soc., Diu. Fuel. Chem. 1993, 38 (l), 131. ( 2 ) Farcasiu, M.; Charlene, M. US Patent 1991, 5,061,363. (3) Wills, J. G. Lubrication Fundamentals;Marcel Dekker, Inc.: New York, 1980.
in the oil during coprocessing of coal with a heavy oil has been found to be due to the deposition on the coal residue or pitch.' The demetallation of used oil during hydrotreatment was also found to be primarily due to the process of physical deposition on the solid catalyst bed.6 The use of unsupported dispersed catalysts for conversion of coal to liquids via direct coal liquefaction is believed to be a very effective method to overcome the limitations of supported metal catalysts? The restricted access to the reaction surface of the supported metal catalysts such as CoMo/AlzOa used in direct coal liquefaction prevents them from influencing the reactions of coal and high molecular weight coal-derived products.6 In addition, supported metal catalysts suffer from rapid deactivation.6 Unsupported dispersed catalysts provide efficient contact of coal/solvent slurries with the catalyst surface.6 The effective dispersion of the catalysts can be achieved by different methods such as using water-soluble' or oil-soluble precursors* and by using finely divided powder^.^ These techniques allow formation of the active inorganic phase under reaction conditions. The addition of finely divided solid precursors with high specific surface area is considered a very effective way to achieve good dispersion and improved overall coal conversion and selectivity to oil production in direct coal liquefaction.gJ0 Iron-based catalysts have the potential to be used as effective dispersed catalysts and have been employed recently for direct coal liquefaction. Iron-based catalysts are inexpensive,readily available, and disposable. Sulfated iron oxide was found to be an effective catalyst for liquefaction with 86 wt 5% conversion and 50 wt % ~~~
(4) Miller, T. J.; Panvelker, S. V.; Wender, I.; Shah, Y. T.; Huffman, G. P. Fuel Process. Technol. 1989,23, 23-38. (5) Skala, D. U.; Saban, M. D.; Orlovic, M.; Meyn, V. W.; Severin, D. K.; Rahimian, I. G. H.; Marjanovic, M. V. Znd. Eng. Chem. Res. 1991,30, 2059-2065. (6) Derbyshire, F. CHEMTECH. 1990, July, 439. (7) Cugini, A. V.; Uta, B. R.; Fromell, E. A. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1989. (8)Herrick, D. E.; Tierney, J. W.; Wender, I.; Huffman,G. P.; Huggins, F. E. Energy Fuels 1990,4, 231. (9) Marriadassou, D. G.; Charcosset, H.; Andres, M.; Chiche, P. Fuel
-1983. - - -, 62. - -, 69-72. - - .-. (10) Suzuki, T.; Yamada, H.; Sears, P.; Watanabe, Y. Energy Fuels 1989, 3,707-713.
0887-0624/94/2508-0099$04.50/00 1994 American Chemical Society
Sanjay et al.
100 Energy &Fuels, Vol. 8, No. I, 1994
Table 1. ProDerties of Waste Oil specific gravity 0.888 ash content sulfur content 1.01 % Metal Analysis metal PPm metal ~
A1 As
Ba Ca co Cr cu K
2.55 1.24 2.14 1024 0.1 0.15 10.3 0.81
MI3 Mn
Mo
P Pb Si Zn
0.45%
PPm 385.45 2.13 0.49 843.09 1.74 4.78 803
selectivity for oils.ll The addition of elemental sulfur to the catalyst was found to further increase the conversion and selectivity. It was postulated that the sulfate group inhibits agglomeration of the metal oxides and subsequently increasesthe surface area and catalyst dispersion.ll Oil-soluble iron carbonyls have been used in direct coal liquefaction and in coprocessing with heavy oil in a number of studies.*J2-15 The iron carbonyls are distributed throughout the coal/solvent mixture and decompose upon heating to form very small catalyst particles active for liquefaction of coal. The addition of sulfur in either elemental form or as an organic sulfur compound favored the formation of pyrrhotite, whereas the less active iron oxide (Fe30r) was formed in its absence.13 The iron pentacarbonyl precursor was converted to pyrrhotite at the reaction conditions with time. The use of 0.5 wt % iron as iron pentacarbonyl increased the coal conversion from 39 to 82% Hematite (Fe203) was found to be a very good sulfur scavenger during coal desulfurization.16 The iron oxide reacts with all the hydrogen sulfide released to form pyrrhotites and thus prevents any reaction of hydrogen sulfide with the organic constituents of the process solvent. The formation of pyrrhotites as the major phase has also been reported when iron oxide was presulfided in a mixture of hydrogen and hydrogen sulfide under reaction conditions.17
Experimental Section Starting Materials. High-volatilebituminous (hvAb)Blind Canyon coal was obtained from the Penn State coal bank. The coal was crushed and separated to obtain a particle size of less than 14 mesh before use. Waste lubricating oil was obtained from a Department of Defense (DoD) installation and was used as received. The properties of the waste oil and metals analysis are given in Table 1. Tetralin (99+% pure) was obtained from Fisher Scientific Co. and was used as received. The catalyst precursor used for coprocessing was superfine iron oxide (SFIO) supplied by Military/Aerospace Chemicals (MACH I, INC). The amount of iron oxide used was the stoichiometric amount (2.5%) required to remove all the sulfur present in the waste oil (1.0%) before coprocessing. Tubing Bomb Microreactor (TBMR).The coprocessing reactions in the tubing bomb were carried out using DECS 6 (11) Pradhan, V. R.; Tierney, J. W.; Wender, I. Energy Fuels 1991,5,
-". ""..
AQ7-M7
(12) Suzuki, T.; Yamada, 0.;Fujita, K.; Takegami, Y.; Watanabe, Y. Chem. Lett. 1982, 1467-1468. (13) Suzuki,T.;Yamada, O.;Takehaski,Y.; Watanabe,Y.FuelProcess. Technol. 1986,10, 33-43. (14) Watanabe, Y.; Yamada, 0.;Fujita, K.; Takegami, Y.; Suzuki, T. Fuel 1984,63, 752-755. (15) Montano, P. A,; Stenberg, V. I.; Sweeny, P. J.Phys. Chem. 1986, 90,156-159. (16) Garg, D.; Tarrer, A. R.; Guin, J. A.; Curtis, C. W.; Clinton, J. H. Ind. Eng. Chem. Process Des. Dev. 1980, 19, 572-580. (17) Shridharani, K. G. Ph.D. Dissertation, Auburn University, 1983.
coal, waste oil (1% sulfur, 0.45% ash), tetralin as a solvent (in some cases), and superfine iron oxide (Fez031 as a catalyst precursor. The tubing bomb microreactorswere made of stainless steel tubing with an outside diameter (0.d.) of 1 in.; the inside diameter was 0.875 in., and the length was approximately 12.5 cm. The tube was sealed at one end, while the other end could be closed with a cap after charging the reactanb. A tube of 0.25 in. 0.d. was welded perpendicularly to the tube approximately 9 cm from the bottom. The tube was connected to a fine metering valve for charging gas; the valve could be capped with a Swagelok fitting. Waste oil (6 or 9 g) and coal (0.6 or 0.9 g) were charged in the desired proportions into a tubing bomb reactor. Tetralin (1.2 g) and iron oxide (0.15 g) were also charged into the reactor in some of the experiments. The open end was sealed using a Swagelok cap and high-pressure hydrogen (1200 psig, measured at room temperature) was added through a fine metering valve and capped with a Swagelok fitting. The bomb was then leak tested by submerging it in water. It was next attached to a variable-speed motor via an extension arm, then lowered into a fluidized sand bath to maintain the reaction temperature (400 "C) and shaken vertically (720 or 425 rpm). At the end of the desired reaction time (60 min),the motor was stopped,the tubing bomb removed, and the reaction quenched using water at room temperature. The liquid and solid product mixture was fractionated using solvent extraction, with hexane and tetrahydrofuran (THF) as the solvents. The hexane-soluble fraction was defined as oils fraction and the residue remaining after the THF extraction was used to estimate the coal conversion. The oil fraction (hexane solubles) was analyzed for sulfur and ash content using a LECO sulfur determinator (SC-32) and a SYBROM Thermolyte furnace, respectively. Reactor System. Ajet-loop reactor system was developed in the laboratory to control the free-radical propagation rate and improve the mass transfer in a free-radical reaction such as coal liquefaction. The system was integrated with a unique gas-driven pumping system to operate under severe coal liquefaction conditions. The details of the system are reported elsewhere and are shown in Figure 1.18 The reactor system was used for coprocessing coal with waste oil. The reactor system is located between the low-pressure tank and the high-pressure tank of the pumping system. The driving force for slurry flow through the reactor system is the pressure differential between the two tanks. The valve switching and sequencing for the pumping system are controlled by a programmable logic controller (PLC). The two slurry tanks were first charged with the required amounts of coal and waste oil (1.5 L of waste oil and 150 g of coal in each tank). The tanks are equipped with a heating furnace; the slurry was allowed to heat to 100 "C with constant stirring of the contents at 1400 rpm. The pumping system was switchedon at this point. The high-pressure tank was at 1600 psig and the low-pressure tank was at 1400 psig. The slurry is heated as it flows from one tank through the preheater into the packed reactor tubes placed in a heated fluidized sand bath and back into the other tank for recycle through the system. The residence time for the slurry in the high-temperature reaction zone is approximately 6.0 s per pass. The reactant hydrogen was supplied to the reactor through a nozzle at 1600 psig when the temperature of the slurry coming from the reactor was approximately 250 OC. A typical temperature profile at different points in the system is shown in Figure 2. The difference between the reactor temperature and the sand bath temperature indicates the driving force available for heat transfer to the slurry in the reactor tubes. Sampleswere collected at different temperatures during the heat-up period. The slurry was heated to the desired temperature and all the heaters were switched off when a significant decrease in the hydrogen uptake rate was observed. Decrease in the hydrogen consumption leads to a significant increase of pressure in both the low and high (18) Sanjay, H. G.; Tarrer, A. R. Prepr. Pap.-Am. Fuel Chem. 1993, 38 (2), 626.
Chem. Soc., Diu.
Energy & Fuels, Vol. 8, No. 1, 1994 101
Iron-Based Catalysts
I
1,
+-2
VS1
t
I
-
v53
t
tl
-Tu
-
II
Tnk 1
425
1
400
j
Figure 1. Jet-loop reactor system with gas-driven pumping system.
450
.!.
Reactor Inlet Reactor Outlet oooOo SandBath
/
X=
wR-
wC-
wash
Wmoisture ash free coal
350 I
and conversion to hexane solubles (H) was defined in a similar manner. The selectivity was defined as the ratio of conversion to hexane solubles and conversion to T H F solubles i.e.,
325 I
selectivity = 100s
375 f
300 f
where
250v
S=H/Y and W Ris the weight of residue remaining after solvent wash (the residue was dried in an oven overnight a t 90 "C before weighing), WCthe weight of catalyst (it was assumed that all the iron oxide was converted to FeS), and WaBh the weight of ash in the coal. Conversion and Selectivity. The conversion of coal and the selectivity for the formation of oils was measured for coprocessing coal with waste oil. The selectivity as defined above is the fraction of total products from coal that are soluble in hexane. The waste oil was completely soluble in hexane before the reaction. I t was assumed that the residue remaining after the solvent wash of the products was primarily the unconverted coal, ash and the catalyst (when used). The reproducibility of the results was checked periodically and was found to be f2 5%. Tubing Bomb Microreactor. (i) Effect of Tetrcalin and Catalyst. The conversion of coal and selectivity for
275 {
250 1 3
225 7
I
conversion (Y)= lOO(1- X) where
/
200-l~~II~11III1IIIIIIIIIIIII1IIIr 200 0 20 40 60 80 100 120 Tlme, min Figure 2. Temperature profile in the jet-loop reactor system.
pressure tanks. The heaters were switched off when a significant increase in the pressures was noted, but the slurry recycling was continued until the temperature of the slurry from the reactor decreased to approximately 175O C . A samplewas collected before the system was drained. Results and Discussion The coal conversion to THF solubles during coprocessing with waste oil was defined as
102 Energy & Fuels, Vol. 8, No. 1, 1994
Sanjay et al. NTHF SOLUBLES
SELECTIVITY
%
HEXANE SOLUBLES
100 80
60 40
20
z -1
U
cc
IIW /
0
1.3
3.0
6.0
DISPERSANT, %
Figure 3. Effect of tetralin and catalyst precursor on coal conversion and selectivity during coprocessing (6 g of waste oil, 0.6 g of DECS 6 coal, 400 "C, 1200 psig of hydrogen pressure at room temperature, shaking speed 720 rpm, 60 min reaction time; with tetralin: 9 g of tetralin, 4.5 g of DECS 6 coal, same reaction conditions).
E3Conversion %
I
1oo/l
U -
5
15
30
90
Time, min Figure 4. Effect of reaction time on coal conversion and selectivity during coprocessing (9 g of waste oil, 0.9 g of DECS 6 coal, 400 "C, 1200 psig of hydrogen pressure at room temperature, shaking speed 720 rpm).
oils was found to be very high (85and 70 5% ,respectively) without the addition of a donor solvent (tetralin) and catalyst precursor. This is attributed to efficient dispersion of coal fragments, due to the presence of dispersant additives in waste oil. The conversion increased slightly with the addition of either the catalyst precursor or tetralin, but the selectivity change was not significant under the conditions of this study (Figure 3). The results from a blank run with coal in tetralin is also shown in Figure 3. The results indicate that the conversion with tetralin as the solvent was higher as expected because tetralin is a better hydrogen donor solvent compared to waste oil. (ii) Effectof Time. The effect of time on the yield from coprocessing was studied without the addition of tetralin and iron oxide. The conversion a t the end of five minutes is only 50%, but the selectivity is relatively higher. The results are shown in Figure 4. The figure shows that the conversion and selectivity do not change significantly after a reaction time of 30 min. The selectivity in all cases is
Figure 5. Effect of dispersant concentration on coal conversion (10 g of mineral oil, 3 g of DECS 17 coal, 0.1 g of sulfur, 0.06 g of FezOa, 400 "C, 1200 psig of hydrogen pressure at room temperature, shaking speed 425 rpm, 60 min reaction time).
consistently high (more than 75%), and remains fairly constant indicating that the coal is primarily converted to oils during coprocessing. (iii) Effect of Dispersion. The role of a dispersant in coal liquefaction was evaluated using a mineral oil (paraffinic oil with no additives) as the solvent and with different concentrations of a commercial dispersant additive (HiTEC 7049, a proprietary mixture supplied by Ethyl Corp.) added to the reaction mixture. Elemental sulfur (0.1 g) was used as the sulfur source for the iron oxide precursor (0.06 g) in these experiments. The conversions were higher in the presence of the additive and increased with the concentration of the additive, but the selectivity decreased. However, a relatively lower increase in the conversion to hexane solubles was observed with the additive concentration (Figure 5). The primary role of the dispersant additive is believed to be to increase coal dispersion and dissolution. This can be seen by the increased conversion with the additive concentration, but a relatively lower increase in the conversion to hexane solubles. The selectivity and the conversion to hexane solubles can be increased by enhancing the gas-liquid mass-transfer rate and better control of the free-radical propagation rate. The jet-loop reactor system described earlier was designed for this purpose and was used for coprocessing coal with waste oil. The coprocessing results from the loop reactor system are discussed in the next section. The shaking speed (425 rpm) of the TBMR during coal liquefaction with mineral oil and the dispersant additive was lower than that used with waste oil (720 rpm). The gas-liquid mass-transfer limitations are lowered as the shaking speed is increased. A lower speed was used during liquefaction with the mineral oil to study the effect of the dispersant under conditions which could be mass-transfer limiting. The conversion of coal in the mineral oil with a dispersant additive is comparable to the conversion with waste oil. However, the conversion to hexane solubles and selectivity are higher with the waste oil. This is attributed in part to the higher shaking speed used while evaluating coal coprocessing with waste oil. This supports the hypothesis that the dispersant additives present in
Energy & Fuels, Vol. 8,No. 1, 1994 103
Iron-Based Catalysts
Table 2. Sulfur and Ash Content of Hexane-Soluble Fraction after Comocessinp
CONVERSION SELECTIVITY
%
2.50
-
-
20
~~
5 0.79 0.20 15 0.78 0.13 30 0.73 0.09 60 0.642 0.07 60 0.712 0.05 60 0.617 0.13 400 "C, 1200 psig (at room temperature).
a Reaction conditions: Catalyst precursor: superfine iron oxide (Fe&). of waste oil.
-20
275
306
320 "C
REACTOR OUTLET TEMPERATURE
Figure 6. Conversion and selectivity of coal during coprocessing in t h e jet-loop reactor system.
the waste oil could play a significant role in improving the coal conversion. The results from this initial study with waste oil in the TBMR indicate the potential beneficial effects of coprocessing. The improved dispersion of the coal particles in the TBMR resulted in higher conversions (>80%). The ratio of coal to waste oil was limited to 10%for most part of this study. A few runs with higher coal concentrations (20 and 25%) resulted in a relatively lower conversion (60%) and selectivity (50%). Reactor System. The samples collected in the reactor system during heat-up were used to determine the conversion of coal and selectivity for oils. The results are shown in Figure 6. A t lower temperatures (275 "C), the conversion was lower, but the conversionto hexane solubles was negative as indicated by the negative selectivity. This was attributed to significant solvent (waste oil) incorporation into the coal. Solvent incorporation at lower temperatures has been reported by a number of investigators.l9 However, when the temperature was increased to approximately 305 "C, the coal conversion was 15%, but the conversion to hexane solubles was higher as indicated by the high selectivity (90%1. As the temperature increased to 320 "C, the conversion increased to nearly 50% with a selectivity of over 95%. In one coprocessing experiment, the temperature of the slurry coming out of the reactor was increased to 390 "C and resulted in coal conversion of 83%, with a selectivity of 95%. The conversion of coal and the high selectivity in the loop reactor system a t relatively lower temperatures (
