Kinetics of Coal Liquefaction at Very Short Reaction Times - American

Energy & Fuels 1996, 10, 641-648

641

Kinetics of Coal Liquefaction at Very Short Reaction Times He Huang, Keyu Wang, Shaojie Wang, Michael T. Klein, and William H. Calkins* Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received December 15, 1995. Revised Manuscript Received February 23, 1996X

Kinetics of thermal and catalyzed coal liquefaction was investigated from 10 s to as long as 60 min using a short contact time batch reactor (SCTBR). Using this reactor system avoids the problems of slow heat up and cool down associated with the massive apparatus required for running high-pressure and high-temperature reactions. Coal liquefaction conversion to tetralinsoluble products was determined by changes in ash content in the partially reacted coal relative to that of the unreacted coal. Three distinct phases in the liquefaction process were observed: a very rapid extraction followed by an induction period (i.e., a short period when the extraction is ending and the conversion appears to be stopped) and then a slower conversion of the coal structure itself. By examining the liquefaction process at very short reaction times, it is possible to separate the very rapid extraction process (representing 20-30 wt % of the total conversion) from the slower liquefaction of the coal structure itself. Most previous work has measured the overall reaction rate by studying the process to higher or complete conversion, resulting in a composite of several vastly different rate processes and confounding a valid kinetic analysis. Solvents with different hydrogen donor strengths were also shown to have a strong influence on conversion rate and molecular hydrogen consumption. Thermogravimetric analysis was used to measure the contents of volatile matter, fixed carbon, a possible precursor or intermediate to the retrograde product formation (tar and coke), and ash in the liquefaction solids as a function of liquefaction time.

Introduction The direct liquefaction of coal is a complex combination of physical and chemical processes.1-3 Initially, extractable material is removed from the coal by the process solvent in an amount that depends upon the coal and the solvent characteristics at the temperature of the process. In the subsequent chemical processes, chemical bonds are cleaved thermally or catalytically to form lower molecular weight products. In competition, bond-forming (retrograde) reactions of the coal and coal liquefaction intermediates lead to high molecular weight products. These products can foul catalysts, plug the reactor system, and otherwise obscure the underlying chemical fundamentals. This motivated the investigation of the coal liquefaction process at very low conversions, where secondary and retrograde reactions are minimized and the initial liquefaction products can be isolated and studied. The elucidation of all these processes and their kinetics are important if further progress is to be made toward improving the yields, productivity, and product quality of the direct liquefaction process. The liquefaction of coal occurs at relatively high temperatures and involves free-radical processes. Such free-radical processes involve the usual initiation, propagation, and termination of these radicals. However, they also involve secondary reactions with the initial Abstract published in Advance ACS Abstracts, April 1, 1996. (1) Whitehurst, D. W.; Mitchell, T. O.; and Farcasiu, M. Coal Liquefaction; Academic Press: New York, 1980. (2) Gorin, E. Chemistry of Coal Utilization Second Supplementary Volume; Elliott, M., Ed.; Wiley Interscience: New York, 1981. (3) Derbyshire, F. J. “Catalysis in Coal Liquefaction: New Directions for Research”; IEA CR/08, ISBN 92-9029-158-3, June 1988.

products, which are particularly important because of the formation of the retrograde products. Although progress in improving the direct coal liquefaction process over the past decade has been particularly significant,4 the fundamentals underlying the physical and chemical processes during the coal liquefaction are still incompletely understood. This is especially true of the initial processes involved in coal liquefaction. It has been difficult, in the past, to produce well-defined, short reaction time samples for in-depth analysis. For those reasons, it is necessary to investigate the liquefaction process at the very early stages before the secondary reactions become particularly significant. This motivated the development of a short contact time batch reactor (SCTBR)5 for the investigation of the liquefaction kinetics and mechanisms at very short contact times. With this reactor system, it is possible to study the process from the very early stages before secondary reactions become important. This reactor has also provided the capability of studying the kinetics of liquefaction under a wide variety of conditions (temperature, pressure, solvent, catalysts, etc.) from very short contact times (0.01-10 min) to practical operation times (30-60 min). Another important step in the development of a system for the study of the coal liquefaction process is the use of thermogravimetric analysis on the coal liquids and the partially reacted coal residues.6 This TGA

X

0887-0624/96/2510-0641$12.00/0

(4) McGurl, G. V.; Lee, S. R.; Srivastava, R. D. Proc. 1993 Pittsburgh Coal Conf. 1993, 270-276. (5) Huang, H.; Calkins, W. H.; Klein, M. T. Energy Fuels 1994, 8, 1304-1309. (6) Huang, H.; Wang, K.; Calkins, W. H.; Klein, M. T. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1994, 39(3), 741.

© 1996 American Chemical Society

642

Energy & Fuels, Vol. 10, No. 3, 1996

Huang et al.

technique allows study of the liquefaction products of the process from a thermal analysis standpoint and provides information concerning the retrograde processes. Experimental Section Apparatus. The study of coal liquefaction under the typical process conditions of high-temperature and high-pressure requires relatively massive equipment. Such equipment has a high heat capacity and is therefore slow to heat up and cool down. This makes a kinetic study at very low conversion quite difficult. To avoid these problems, a short contact time batch reactor (SCTBR) was devised which allows the heat up of the process stream to reaction temperature in about 0.3 s. The removal and quenching of the reaction products occurs in a similar time period. The design and operation of such a SCTBR reactor system including a schematic diagram have been described in detail elsewhere.5,7 In brief, the 30 cm3 reactor is constructed of 3/4 in. o.d. stainless steel tubing of approximately 12 in. length with a wall thickness of approximately 0.433 in. The 21 ft lengths of coiled 316 stainless steel tubing used for both the preheater and precooler are 1/4 in. o.d. with wall thickness of 0.035 in. The reactor system is capable of containing up to 17 MPA (2500 psi) at temperatures up to 550 °C. In operation, both the empty preheater and the reactor are immersed in a Techne IFB-52 fluidized sand bath. They are brought up to the predetermined reaction temperature prior to the start of the liquefaction. High-pressure hydrogen or nitrogen gas provided the driving force to deliver the slurry mixture of coal-solvent or coal-catalyst-solvent under study from a small blow case at ambient temperature into the empty reactor through the preheater tubing. Hydrogen or nitrogen gas was then bubbled through the reactor from the bottom to provide the agitation needed in the heterogeneous liquefaction reaction. The degree of agitation is controlled by the exit-gas flow rate from the top of the reactor. A small water-cooled condenser and disengaging space to improve operability and to prevent loss of solvent or other low-boiling products produced in liquefaction are located above the reactor and before the let-down valve. In the case of running under hydrogen pressure, the gas bubbles are also used to supply the hydrogen for the liquefaction reactions. By this means, very high contact or interfacial surface between the gas and the slurry reaction mixture is provided. The temperature of the reactants (ca. 30 g), initially at ambient temperature, approaches the desired reaction temperature to within 5-8 °C during the transport process (approximately 0.3 s) and reaches the predetermined reaction temperature within 30 s. The temperature is maintained within (2 °C during the liquefaction. The rapid heat up and stable temperature profile are due to both the small quantity of the reaction mixture relative to the massive reactor and turbulent flow of the reactants through the preheater. At a preselected time, the high-pressure gas is again used to drive the reactor contents from the reactor into a cold receiver through the precooler. Both receiver and precooler are immersed in a water bath. Quenching of the product mixture to about 25 °C is achieved during the transport process (approximately 0.3 s). An illustrative temperature-time profile for the Illinois No. 6 coal liquefaction in tetralin at 390 °C is shown in Figure 1. The dashed lines represent the estimated temperatures of the slurry mixture during the transport through the preheater or the precooler by mathematical modeling.7 Thermogravimetric Analysis. The thermogravimetric analyzer (TGA) was a Model 51 TGA (TA Instruments, New Castle, DE). An approximately 30 mg sample of the coal or (7) Huang, H.; Fake, D. M.; Calkins, W. H.; Klein, M. T. Energy Fuels 1994, 8, 1310-1315.

Figure 1. Temperature-time profile for the Illinois No. 6 coal liquefaction under 1000 psig of N2 in tetralin at 390 °C for 120 s. coal residue was loaded in a quartz pan and mounted in the instrument. The program of manipulation of the TG variables was determined by the objectives of the particular experiment. A representative TG scan on the Argonne Premium Illinois No. 6 bituminous coal, which was dried in a vacuum oven with a nitrogen purge at 105 °C for 48 h before use, is shown in Figure 2. The weight loss resulting from heating in nitrogen at 100 cm3(STP)/min with a heating rate of 10 °C/min to 950 °C and hold for 7 min at 950 °C defines the amount of volatile matter (VM). The first weight loss up to 185 °C (about 1.8 wt %) represents the residual moisture in the sample. For this illustrative sample VM was 35.5 wt % on dry basis. Further weight loss occurred at 950 °C after the introduction of oxygen. This was due to the oxidation of the remaining combustible material, the so-called fixed carbon (FC), in the char, which amounted to 49.3 wt % on a dry basis for the illustrative sample. The residue represents the ash content (15.2 wt % on a dry basis), which is in agreement with that determined by ASTM D3174. In summary, the two phases, i.e., (1) the heating rate to 950 °C in nitrogen and hold for 7 min and (2) the oxidation at 950 °C, provided measures of VM, FC, and ash. The experimental error for determination of these TGA characteristic parameters (VM, FC, and ash) is less than (2% of the measured values. The differential of the weight loss (DTG) curve highlights the various TG processes more clearly. The DTG curve for Illinois No. 6 shows a pattern which is more complex than many of the other Argonne coals. This becomes even more distinct and complex if the heating rate is slowed down to about 1 °C/min.8 Catalyst Used. Molybdenum naphthenate (6.8 wt % molybdenum from Shepherd Chemical Co.) was the liquefaction catalyst used in this study. The catalyst was prepared by dissolving about 0.5 g of molybdenum naphthenate in tetralin, which is equivalent to about 0.9 wt % Mo based on the amount of the coal used in each run. The catalyst was then sulfided by reacting the solution with about 1 g of methyl disulfide during the transport and liquefaction. Coal Liquefaction. Illinois No. 6 bituminous coal and Wyodak Anderson subbituminous coal from the Argonne Premium Coal Sample program have been used for this study. Proximate and elemental analyses of these coals are shown in Table 1. More analytical data are available in the User’s Handbook for the Argonne Premium Coal Sample Program.9 All liquefactions were run as mixtures of tetralin (T, the H-donor solvent) and coal (C) at a ratio of T/C ) 8 to minimize (8) Huang, H.; Wang, K.; Calkins, W. H.; Klein, M. T. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1995, 40(3), 465. (9) Vorres, K. S. User’s Handbook for the Argonne Premium Coal Sample Program, ANL/PCSP-93/1.

Kinetics of Coal Liquefaction

Energy & Fuels, Vol. 10, No. 3, 1996 643

Figure 2. TG scan on the Illinois No. 6 bituminous coal at 10 °C/min. Table 1. Proximate and Elemental Analyses proximate nalysis,a wt % coal sample

VM

FC

ash

elemental analysis,b wt % C

H

O

S

N

Illinois No. 6 40.1 44.5 15.5 77.7 5.00 13.5 2.38 1.37 Wyodak Anderson 44.7 46.5 8.8 75.0 5.35 18.0 0.47 1.12 a

Dry basis. b Daf basis.

Figure 4. Removal of the residual tetralin from the liquefaction residues using methylene chloride rinsing. The control sample is the original coal which is processed exactly as a liquefaction residue except at ambient temperature.

the effect of changing donor solvent concentration during the reaction. About 4 g of coal was used for each reactor run, together with added tetralin to make up the reactant slurry. Hold up of material on the surfaces of the reactor, preheater, and precooler prevented complete recovery of the reaction products. However, the recoveries were high and varied from about 80 to 90 wt %. The measure of conversion and subsequent analytical results were based on representative aliquots.5,10,11 The reactor system is cleaned in place by a series of tetralin washes. The wash effectiveness was followed by analysis of

both soluble reactant concentration and particulate concentration in the wash streams, which were monitored by gas chromatography and filtration, respectively. Workup Procedures of Reaction Products. After a liquefaction run, the product mixture was filtered and the solid residue washed with cold fresh tetralin thoroughly and dried in a vacuum oven with a nitrogen purge at about 105 °C for 48 h. This resulted in the production of a liquid filtrate, which consisted mainly of tetralin and dissolved coal liquids, and a solid filter cake, which consisted of unconverted and/or partially converted solid coal residue. These two fractions are then analyzed and processed separately, following the schematic diagram in Figure 3. This paper focuses on the analytical results of the solid filter cake. The DTG curve (the derivative of the TG) of the dried solid residue sample, together with the unreacted original coal sample and the dried solid residue rinsed by cold methylene chloride to remove residual tetralin, is shown in Figure 4. Determination of Liquefaction Conversion. During coal liquefaction, most of the coal liquids are extracted into the tetralin solvent. However, the mineral matter of the coal remains with the solid coal residue and is insoluble in tetralin, as shown by the TG scan on the liquid filtrate.10,11 This

(10) Huang, H.; Calkins, W. H.; Klein, M. T. Ind. Eng. Chem. Res. 1994, 33, 2272-2279.

(11) Calkins, W. H.; Huang, H.; Klein, M. T. Proc. 1994 Pittsburgh Coal Conf. 1994, 475-480.

Figure 3. Schematic diagram of the experimental procedure.

644


Huang et al.

Figure 6. (a) VM (volatile matter) and (b) FC (fixed carbon) in the thermal (uncatalyzed) liquefaction residues determined by TGA. liquefaction: tetralin:coal ) 8:1; 1000 psig of N2. TGA: 100 cm3(STP)/min N2; 100 °C/min.

Figure 5. Liquefaction conversion of the Illinois No. 6 coal under 1000 psig of N2 (tetralin:coal ) 8:1): (a) at short contact times; (b) for up to 60 min. property provides a means of determining the conversion of the coal to tetralin-soluble oils by ash content of the solid coal residue when it has been washed free of the coal liquids, tetralin, and tetralin-derived products. Since the ash is not consumed during the liquefaction and remains in the solid, conversion to the tetralin-soluble oils can be derived based on ash balance.10,11 This provides a working equation

(

conversion ) 1 -

)

Ao × 100% As

(1)

where Ao and As are the weight fractions of ash in the control sample and in the liquefaction residue, respectively. The control sample is the original coal which is processed exactly as a liquefaction residue except at ambient temperature. The physical extraction of the soluble fraction of the coal into the cold tetralin (i.e., under ambient temperature) is negligible by GC analysis of the liquid filtrate. When the liquefaction is carried out in the presence of an inorganic catalyst, the conversion calculation must include an ash value corrected for the ash derived from the catalyst. It should be pointed out that these studies were aimed at liquefaction of coal defined as conversion to tetralin soluble products. This represents conversion to products which do not include some asphaltenes or preasphaltenes which would have to be recycled or hydrocracked to useful fuels. If the filter cake from a liquefaction run is extracted with tetrahydrofuran, an additional extraction of 10-15 wt % is obtained fairly uni-

formly over the conversion range studied (see ref 11). This represents some increase in liquefaction yields if the THFsoluble tetralin-insoluble materials are included. For the purpose of this kinetic study, however, tetrahydrofuran extraction was not included in the standard workup to avoid the arbitrary separation of liquefaction products on the basis of solubility in different solvents. Data Processing. All graphs of conversion data and TGA parameters (VM and FC) were plotted by least squares curvefitting to show trends in the data.

Results and Discussion Thermal (Uncatalyzed) Liquefaction. The conversion curves of thermal liquefaction (without added catalyst) of Illinois No. 6 in tetralin run under 1000 psig of nitrogen at three temperatures (390, 408, and 422 °C) are shown in Figure 5, a and b, for two different time intervals. Three distinct phases in the process were observed, i.e., a very rapid conversion (from 20 to 30%) followed by a short “induction” period during which the conversion does not change significantly and then a slower liquefaction of the coal structure. The initial rapid conversion in the first minute is due to the physical extraction of a soluble fraction of the coal into the hot tetralin. This is followed by an induction period and then the slow conversion of the coal structure to liquid products. As the liquefaction temperature increases, the amount of extraction increases and the induction period becomes shorter. In the absence of a catalyst, however, increases in temperature above 408 °C result in little increase in soluble products. The reason for this is found in the TG analysis of the solid residue. The rate of removal of the volatile matter increases steadily as the temperature increases (Figure 6a), regardless of whether the liquefaction is in hydrogen or in nitrogen. However, the fixed carbon in the



Table 2. Elemental Analysis of the Illinois No. 6 Coal Liquefaction Residuesa liquefaction run sample

T (°C)

t (min)

gas

DOE090 DOE095 DOE089 DOE088 DOE096 DOE091 DOE093 DOE092 DOE094 DOE104 DOE105 DOE097 DOE100 DOE099 DOE098 DOE101 DOE102 DOE103 DOE106 DOE107 DOE108 DOE109

407 411 406 408 410 408 409 406 409 423 423 426 423 424 424 422 422 421 389 388 389 388

0.50 0.75 1.00 2.00 3.00 5.00 10.00 30.00 60.00 0.50 0.75 1.00 2.00 3.00 5.00 10.00 30.00 60.00 5.00 5.00 5.00 5.00

N2 N2 N2 N2 N2 N2 N2 N2 N2 N2 N2 N2 N2 N2 N2 N2 N2 N2 N2 H2 N2 H2

elemental analysis catalystb

yes yes yes yes

SAb

C

H

N

S

H/C (atomic)

yes yes

65.04 65.16 64.35 63.76 63.75 63.72 63.16 63.13 63.73 65.84 65.71 64.77 65.24 65.76 64.88 64.14 64.63 65.21 65.81 64.89 64.68 63.33

4.33 4.33 4.22 4.14 4.08 3.97 3.76 3.71 3.46 5.06 4.66 4.09 4.18 4.06 3.97 3.83 3.18 2.97 4.38 4.47 4.23 4.60

1.38 1.31 1.45 1.43 1.40 1.40 1.41 1.44 1.48 1.40 1.44 1.39 1.45 1.46 1.46 1.48 1.51 1.48 1.34 1.35 1.39 1.37

3.85 3.91 3.68 3.77 3.61 3.51 3.26 3.36 2.80 3.29 3.36 3.55 3.19 2.87 3.05 3.81 2.63 2.60 3.00 3.23 3.61 3.73

0.799 0.797 0.787 0.779 0.768 0.748 0.714 0.705 0.651 0.922 0.851 0.758 0.769 0.741 0.734 0.717 0.590 0.547 0.799 0.827 0.785 0.872

a Liquefaction solvent, tetralin; gas pressure 1000 psig. b Catalyst: molybdenum naphthenate, 0.9 wt % of Mo on the basis of coal charged. SA (sulfiding agent): methyl disulfide, about 1 g.

Figure 7. Liquefaction conversion of the Illinois No 6 bituminous under 1000 psig of H2 and under 1000 psig of N2 at 390 °C (tetralin:coal ) 8:1).

Figure 8. Liquefaction conversion of the Wyodak Anderson subbituminous under 1000 psig of H2 and under 1000 psig of N2 at 390 °C (tetralin:coal ) 8:1).

solid residues increases at a very rapid rate above 408 °C (Figure 6b) at the expense of liquefaction yield. The fixed carbon (the nonvolatile fraction) may be an indicator of the retrograde products which result in low liquid yields and the formation of tar and coke. This is also supported by elemental analyses of the solid residues from uncatalyzed liquefaction experiments over a range of times in which the hydrogen to carbon atomic ratio rapidly decreases as the reaction time increases (see Table 2). A similar liquefaction kinetics of the Illinois No. 6 coal was examined in the presence of hydrogen. As shown in Figure 7, an extraction phase and an induction period followed by a slower breakdown of the coal structure were determined. Compared to nitrogen, the conversion in hydrogen was somewhat higher throughout the liquefaction. The same kinetic experiments have also been carried out on the Wyodak Anderson subbituminous coal. The thermal liquefaction conversion curves of this subbituminous coal in hydrogen and in nitrogen are shown in Figure 8. The results show similar liquefaction kinetics to those of the Illinois No. 6 coal. An extraction phase and an induction period followed by a slower breakdown

of the coal structure were also observed. However, unlike the liquefaction of Illinois No. 6, there was little difference in the liquefaction rate in the presence or absence of hydrogen. This is apparently due to the very low concentration of pyrite (or more likely pyrrhotite derived from the pyrite) in the Wyodak Anderson coal. The contents of pyritic sulfur in Illinois No. 6 and Wyodak Anderson are 2.81 and 0.17 wt %, respectively. This is a strong indication that pyrite in the Illinois No. 6 provides some catalysis for the liquefaction in the presence of hydrogen. The induction period observed in the thermal liquefaction of the both coals under the pressure of either nitrogen or hydrogen is actually a pseudo-induction period. This pseudo-induction period is a transition interval which is due to the simultaneous occurrence of two processes, a very rapid extraction and a relatively slower liquefaction of the coal structure. As the temperature increases, the pseudo-induction period steadily becomes less pronounced as the rate of break down of the coal structure increases and becomes closer in reaction rate to the extraction step itself. That the pseudo-induction period is not due to the build up of

646


Huang et al. Table 4. Influence of Solvent with Different Hydrogen Donor Strengths on Direct Coal Liquefaction

Figure 9. DTG profiles for the liquefaction residues of the Illinois No. 6 bituminous at the selected contact times. Liquefaction: tetralin:coal ) 8:1; 390 °C; 1000 psig of N2. TGA: 100 cm3(STP)/min N2; 10 °C/min. Table 3. Catalysis of Coal Liquefaction by Molybdenum Naphthenate molybdenum naphthenate (g)

a

methyl disulfide (g)

Mo (wt %)

conversion (wt %)

0.00 0.00 0.59 0.62

Under 1000 psig of Nitrogen Gas 0.00 0.00 1.03 0.00 0.00 0.85 1.07 0.86

28.1 27.5 28.0 28.4

0.00 0.00 0.61 0.61

Under 1000 psig of Hydrogen Gas 0.00 0.00 1.03 0.00 0.00 0.87 1.15 0.87

33.8 32.3 33.5 41.8

Illinois No. 6; T/C ) 8; 390 °C; 5 min.

free radicals is shown by ESR data.12,13 These ESR results indicate that the radical concentration actually decreases somewhat during the liquefaction process. DTG curves for partially converted coal liquefaction residues of Illinois No. 6 coal after liquefaction in tetralin at 390 °C under 1000 psig of nitrogen at selected contact times are shown in Figure 9. The gradual disappearance of the two smaller peaks (one of them identified as pyrite) clearly indicates that some chemical changes in the solid coal are taking place before the coal actually becomes liquid. Such chemical change during the early stages of liquefaction is supported by previously observed changes in the total oxygen and hydroxyl content of the partially reacted coal.12-14 The Argonne Illinois No. 6 coal used in most of this work is -100 mesh particles. To determine whether the liquefaction rates or the pseudo-induction periods were limited by mass transfer and/or heat transfer, another Illinois No. 6 coal obtained from Amoco, which was much finer, was investigated. The mesh analysis of the two coals is shown in Figure 10. The conversion vs time of this Amoco Illinois No. 6 coal liquefaction is shown in Figure 11. Little, if any, difference can be seen in the conversion rates or the pseudo-induction periods of these two coals, suggesting that, if the coal is finer than 100 mesh, mass and/or heat transfers will not be (12) Huang, H.; Provine, W. D.; Jung, B.; Jacintha, M. A.; Rethwisch, D. G.; Calkins, W. H.; Klein, M. T.; Dybowski, C. R.; Scouten, C. G. Proc. Int. Conf. Coal Sci. 1993, 1, 266. (13) Provine, W. D.; Jung, B.; Jacintha, M. A.; Rethwisch, D. G.; Huang, He; Calkins, W. H.; Klein, M. T.; Scouten, C. G.; Dybowski, C. R. Catal. Today 1994, 19(3), 409. (14) Jung, B.; Provine, W. D.; Calkins, W. H.; Klein, M. T.; Scouten, C. G. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1992, 37(2), 670.

solvent

time (min)

T (°C)

1,2,3,4-tetrahydroquinoline tetralin tetralin 1-methylnaphthalene

30 30 10 10

410 408 390 392

in 1000 psig of N2 H2 67.8 49.1 28.6 7.2

67.9 39.7 20.4

controlling. This was also concluded by Guin et al. in their earlier work.15 Catalyzed Liquefaction. Conversion of the Illinois No. 6 coal with molybdenum naphthenate (equivalent to 0.9 wt % Mo) was studied in an effort to understand the role of a hydrogenation catalyst in coal liquefaction. Table 3 summarizes the results of a series of experiments aimed at determining the active species when the molybdenum naphthenate is the added catalyst. The sulfiding agent used in this study was methyl disulfide. The results indicate, not surprisingly, that even sulfided molybdenum naphthenate in the absence of hydrogen is not active. In the presence of hydrogen, only sulfided molybdenum naphthenate (presumable as Mo2S3 or MoS2) is the active catalyst in coal liquefaction. The sulfiding agent itself plays no direct role in coal liquefaction. The liquefaction conversions of Illinois No. 6 coal at 390 °C in the presence of sulfided molybdenum naphthenate catalyst (equivalent to 0.9 wt % Mo) in tetralin under 1000 psig of hydrogen and without any added catalyst in tetralin under 1000 psig of nitrogen are shown in Figure 12. The rapid initial extraction is again observed in the first minute in the catalyzed liquefaction. However, the pseudo-induction period is diminished and the subsequent conversion is much faster for the catalyzed than the thermal liquefaction. More importantly, the retrograde process is very significantly reduced in the presence of catalyst and hydrogen as is shown by the fixed carbon content in the residue as a function of liquefaction time (Figure 13). When the catalyzed liquefaction was run at higher temperature, the fixed carbon of the residue was even further reduced (Figure 14), suggesting that the precursors of the retrograde processes are being hydrogenated and stabilized during the catalyzed liquefaction. Liquid yields were thereby significantly improved (Figure 15). Effect of Solvent on Liquefaction Kinetics. It has been known that liquefaction yields are improved by using a strong hydrogen donor solvent. To demonstrate the dependence of liquefaction on hydrogen-donor ability of a solvent, three solvents, i.e., tetrahydroquinoline, tetralin, and methylnaphthalene, in decreasing order of hydrogen-donor ability, have been used in liquefaction of Illinois No. 6. The results are summarized in Table 4. These data show that the very strong donor solvent tetrahydroquinoline gives much higher conversion than tetralin. Further, the liquefaction conversion in this very strong donor solvent shows no sensitivity to gas atmosphere (nitrogen or hydrogen), indicating little if any hydrogen is derived from the gas phase (e.g., molecular hydrogen) in the case of the use of the very strong donor solvent. However, the very weak hydrogen donor solvent methylnaphthalene gives much lower conversion than tetralin. More importantly, (15) Guin, J. A.; Tarrer, A. R.; Pitts, W. S.; Prather, J. W. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1976, 21(5), 170-179.



Figure 10. Particle size analysis of Argonne Illinois No. 6 coal and Amoco Illinois No. 6 coal.

Figure 11. Liquefaction conversion of the extremely fine Amoco Illinois No. 6 bituminous under 1000 psig of N2 at 394 °C (tetralin:coal ) 8:1).

Figure 12. Conversion of the Illinois No. 6 bituminous in thermal and catalyzed (about 0.9 wt % Mo) coal liquefaction at 390 °C (tetralin:coal ) 8:1).

in this very weak hydrogen donor solvent, the liquefaction conversion in hydrogen is much higher than that in nitrogen, showing strong sensitivity to gas atmosphere. These results suggest that, in a very weak hydrogen donor solvent, the hydrogen needed in the liquefaction process must be mostly derived from molecular hydrogen. Summary and Conclusions The laboratory scale short contact time batch reactor (SCTBR) capable of operation up to 450 °C and 17 MPa

Figure 13. FC (fixed carbon) in the thermal (1000 psig of N2) and catalyzed (about 0.9 wt % Mo and 1000 psig of H2) liquefaction residues determined by TGA. Liquefaction: Illinois No. 6 bituminous; tetralin:coal ) 8:1; 390 °C. TGA: 100 cm3(STP)/min N2; 100 °C/min.

Figure 14. FC (fixed carbon) in the catalyzed (about 0.9 wt % Mo) liquefaction residues determined by TGA. Liquefaction: Illinois No. 6 bituminous; tetralin:coal ) 8:1; 1000 psig of H2. TGA: 100 cm3(STP)/min N2; 100 °C/min.

(2500 psig) provides liquefaction samples at well-defined reaction times from 10 s to 60 min or longer. Using this reactor system, it is possible to study the liquefaction processes and kinetics under a wide variety of conditions. Thermogravimetric analysis (TGA) provides sensitive, rapid, and reproducible results revealing changes in the physical and chemical structure of coal as it undergoes liquefaction. This technique also is capable of measur-

648


Figure 15. Conversion of the Illinois No. 6 bituminous in catalyzed (about 0.9 wt % Mo) coal liquefaction under 1000 psig of H2 (tetralin:coal ) 8:1).

ing the onset and rate of the retrograde reactions occurring during the liquefaction process. A workup procedure for the liquefaction products has been developed. The liquefaction conversion was determined by the use of an ash balance. The ash content was measured using thermogravimetric analysis. Study of the kinetics of coal liquefaction from very short contact times (10 s) to 60 min shows three distinct stages. In the first minute of the liquefaction process, there is a very rapid conversion due to the extraction of tetralin-soluble material from the coal. It is followed by a pseudo-induction period and then a slower liquefaction of the coal structure itself. This process is much more rapid in the presence of hydrogen and a sulfided

Huang et al.

molybdenum naphthenate catalyst. At higher temperatures, the degree of this extraction is higher and the pseudo-induction period shorter. However, in the absence of hydrogen and a catalyst, the breakdown of the coal structure into coal liquid is offset by the buildup of retrograde products. Liquefaction in the presence of hydrogen and sulfided molybdenum naphthenate (a hydrogenation catalyst) resulted in increased liquefaction conversion rate. More importantly, such a system (sulfided catalyst in the presence of hydrogen) results in greatly decreased rate of formation of fixed carbon, a precursor for the retrograde products. A hydrogen atmosphere increases the thermal (uncatalyzed) conversion of Illinois No. 6 but had little effect on Wyodak Anderson subbituminous coal. This may be due to the catalytic effect of pyrite (or pyrrhotite derived from the pyrite) in the Illinois No. 6, since this coal contains substantial amounts of pyrite whereas the Wyodak Anderson contains little pyrite. Liquefaction yields are greatly increased by the use of a strong hydrogen donor solvent in which most of the hydrogen is contributed by the solvent rather than molecular hydrogen. Acknowledgment. The support of this work by the Department of Energy under DE22-93PC93205 is gratefully acknowledged. The use of Argonne Premium Coal Samples provided by Dr. Karl Vorres is also acknowledged. EF950260F

Kinetics of Coal Liquefaction at Very Short Reaction Times - American

Recommend Documents