534
Energy & Fuels 1988,2, 534-538
rate constants are apparent and are considered to be a function of the hydrogen concentration. Further work is needed in the future to evaluate the effective hydrogen concentration during liquefaction reactions. Conclusions 1. Semicoke formed during liquefaction reactions of coal is experimentally defined as a lumped product separated from liquefaction products by hydrogenating the PI fraction in the products. 2. Preasphaltenes play the most important role in retrogressive (semicoke-forming) reactions, and semicoke formation from preasphaltenes occurs through an intermediate species, C*, corresponding to thermal fragments from coal. 3. On the basis of the above mechanism, a comprehensive kinetic model for coal liquefaction accompanied by semicoke formation has been developed, and the mean rate constants of semicoke formation for three different coals have been obtained. The rate constants are on the order of 1.61-3.83 X 1C2min-l and depend on the reaction temperature and coal properties. Acknowledgment. This work was partially supported by a Grant-in-Aid for the Special Project Research on Energy (Energy(1)-No. 58040001 and No. 59040002) from
the Ministry of Education, Science and Culture, Japan. Nomenclature Cclo = weight fraction of component C1 in original coal Cczo= weight fraction of component C2 in original coal CIo = weight fraction of component Io in original coal It = apparent rate constant, min-l y = product yield Components A asphaltenes (BS-HI) benzene-insoluble fraction BI benzene-soluble fraction BS dry ash-free (daf) coal C portion of coal forming O&G consequently via P c1 and A portion of coal forming directly O&G without CZ consuming external hydrogen intermediate from C to P or from P to I C* HI n-hexane-insoluble fraction HS n-hexane-soluble fraction I semicoke fraction defined as PI fraction from rehydrogenation of primary PI fraction portion of coal inherently inert for coal liquefaction O I O&G oils and gases preasphaltenes P pyridine-insoluble fraction PI pyridine-soluble fraction PS
Water and Nondonor-Vehicle-AssistedLiquefaction of Illinois Bituminous Coal? Michael A. Mikita* Department of Chemistry] University of Colorado at Denver, Denver, Colorado 80202
Bradley C . Bockrath,* Henry M. Davis, Sidney Friedman, and Eugene G. Illig Pittsburgh Energy Technology Center, U.S. Department of Energy, Pittsburgh, Pennsylvania 15236 Received November 6 , 1987. Revised Manuscript Received January 26, 1988
Through the use of water or other non-hydrogen-donor vehicles as substitute liquefaction media, high conversions of Illinois No. 6 (River King Mine) bituminous coal were obtained in minireactor experiments at modest temperatures with little or no hydrogen-donor solvents. Typical THF conversions (daf basis) of 67% with water only, 87% with water and SRC 11, and 90%+ with only water and 1000 ppm of Mo were obtained in 30 min at 385 OC with a cold charge of 1200 psig of Hz. A synergism was observed at low ratios (0.5 or less) of donor solvent to coal upon combination of SRC I1 distillate and water. A similar effect was not observed when cyclododecane replaced water. The addition of Mo catalyst precursors to the water allowed complete elimination of donor solvent without loss in conversion. The exclusion of added organic donor solvent permits analysis of the coal-derived liquefaction products without interference. Other experiments were performed for both 20 and 5 min with molecular hydrogen and a donor solvent but with deuterium oxide replacing the water. No isotope effect was found, which suggests that water is not a rate-limiting reactant under the reported conditions. Introduction For a variety of reasons, water has been used in the past in the liquefaction or the extraction of coal. When used in combination with carbon monoxide and a suitable catalyst, water was a source of hydrogen for the reduction
of coal.'s2 In this work, an organic solvent was frequently used in combination with water. Liquefaction under carbon monoxide, without an organic solvent, has also been carried out with slurries composed of coal and either water or aqueous b a ~ e . 9 In ~ some cases, water served to carry
'Reference to a brand or company name is made to facilitate understanding and does not imply endorsement by the U.S.Department of Energy.
(1) Appell, H. R.;Wender, I.; Miller, R.D. Chem. Ind. (London) 1969, 1703. ( 2 ) Appell, H. R. Energy (Oxford) 1976, 1 , 24. (3)Ross, D.S.;Blessing, J. E. Fuel 1978, 57, 379.
0887-0624/88/2502-0534$01.50/00 1988 American Chemical Society
Liquefaction of Illinois Bituminous Coal
Energy & Fuels, Vol. 2, No. 4, 1988 535
dissolved metal salts used as homogeneous catalysts as well as acting as the liquefaction m e d i ~ m . Thus, ~ the elimination of coal-derived recycle solvents commonly used in liquefaction in favor of water has already been accomplished in the laboratory. In comparison with conventional organic liquefaction solvents, water is quite effective when used in combination with H2S, in particular under synthesis gas rather than hydrogen.6 Aqueous liquefaction using impregnated catalysts has also been combined in a single operation with supercritical water distillation to separate the oil and asphaltene from the coal char re~idue.~ The results clearly show that the liquids produced by hydrogenation can be extracted by supercritical water and transported away from insoluble coal residues. The simple treatment of coal with supercritical water in the absence of hydrogen or catalysts renders a substantial portion of the treated coal extractable by tetrahydrofuran after the product is cooled and recovered from the autoclave? The amount of extract obtained depended on the density of the supercritical water. Yields obtained when coal was injected into supercritical water, thus providing a rapid heat up of the coals, were higher than those when a coal-water slurry was heated to operating temperature. From these extraction studies, it is apparent that water is able to assist the diffusion and dispersal of liquefaction products and reactants. In addition to these roles, water may directly participate as a reactant in the thermolytic chemistry of certain model compounds. In the presence of water, dibenzyl ether decomposes at 374 "C by both pyrolytic and hydrolytic pathways, the latter leading to the formation of benzyl alcoh01.~ The removal of nitrogen from heterocyclic compounds, such as isoquinoline, is accelerated in the presence of supercritical water.1° Taken together, these studies indicate that, under various conditions, supercritical water may act as a good liquefaction medium, dissolve or extract coal-derived liquid products, promote the cleavage of certain bonds likely to be found in coal, provide hydrogen through the water gas shift reaction, and possibly assist the contacting of coal with catalysts or hydrogen. The successful preparation of coal-water mixtures for direct combustion makes it clear that pumpable slurries can be made at ambient conditions by using as little as 30% by weight of water. The present study is intended to evaluate further the role of water, near or above its critical temperature, in the conversion of coal to t liquefaction product. Since water is not a hydrogen donor in the usual sense, the behavior of other nondonors was also examined. Note that for coal hydrogenation alone, no vehicle is needed. Much of the previous work on coal hydrogenation was done by simply subjecting dry powdered coal in batch units to a high pressure of gaseous hydrogen, sometimes under the influence of an impregnated catalyst."
Experimental Section A multireactor consisting of five individual microautoclaves, each of approximately 45 mL capacity, attached to a single yoke, was used to study these reactions.12 The entire assembly was (4) Ross, D. S.; Blessing, J. E.; Nguyen, Q.C.; Hum, G.
fi.7 13OR --, -I--.
F. Fuel 1984,
(6)Ross, D. S.; Nguyen, Q. C.; Hum, G. P. Fuel 1984,63, 1211. (6) Stenberg, V. I.; Hei, R. D.; Sweeney, P. G.; Nowok, J. Prepr. Pap.-Am. Chem. SOC.,Diu.Fuel Chem. 1984,29(5), 63. (7) Barton, P. Ind. Eng. Chem. Process Des. Deu. 1983,22, 589. (8) Deshpande, G. V.; Holder, G. D.; Bishop, A. A.; Gopal, J.; Wender, I. Fuel 1984,63,956. (9) Townsend, S. H.; Klein, M. T. Fuel 1985, 64, 635. (10) Houser, T. J.;Tiffany, D. M.; Li, Z.; McCarville, M. E.; Houghton, M.E.Fuel 1986, 65, 827. (11)Hawk, C. 0.; Hiteshu, R. W. 'Hydrogenation of Coal in the Batch Autoclave"; Bull.-US. Bur. Mines 1965, No. 622.
Table I. Elemental Analyses of Coal and Vehicles" anal. material C H N O S Illinois No. 6 coal, River King mineb 73.7 5.6 1.5 14.8 4.5 SRC-I1 distillate 87.1 8.0 1.4 3.0 0.4 "Results are reported as wt. % on a daf basis by Huffman Labs, Wheatridge, CO. bMoisture-free ash content was determined to be 13.6 wt % for the River King Coal. As used, the coal contained 3 wt % water. immersed rapidly into a preheated, fluidized sand bath, allowing heat up to reaction temperature in 4-6 min. Immersion in a second fluidized sand bath held a t room temperature provided rapid quenching. The autoclaves were agitated by a rapid horizontal-shaking motion, assuring good mixing of heterogeneous, multiphase mixtures. Individual thermocouples allowed continuous temperature monitoring of each microautoclave. For all experiments reported here, the reactors, once pressurized, were isolated from the gas-handling manifold by a valve and a short length of tubing of negligible volume. This prevented loss of water from the reaction zone owing to condensation in the unheated portion of the system. Separate experiments using different reactors, in which water could migrate to unheated regions of the system, indicated that such water loss had a profound but erratic effect on measured values for pressure and coal conversion and generally led to misleading data. The pressure at reaction temperature was not measured directly in these experiments. Using the van der Waals equation, the partial pressure of water was calculated to be 1700 psi at 385 "C in those cases where 1.7 g of water was charged. When 3.4 g was charged, the partial pressure was calculated to be about 2600 psi. Total estimated pressures are thus either about 4300 psi or about 5300 psi. The corresponding density of supercritical water was 0.05 g/mL when 1.7 g was charged and 0.11 g/mL when 3.4 g was charged. The analyses of the Illinois No. 6 (River King Mine) bituminous coal and the vehicles used in these experiments are shown in Table I. Most of the conversion values were obtained by the centrifugation method given below. Conversion for run 13 and the three series HD2, HD3, and HD4 were determined by a somewhat different method based on filtration of the insolubles. Separate experiments established that similar values were obtained by both methods. After the gas was collected and measured via water displacement, the reactors were washed out with tetrahydrofuran (THF) and the reactor walls were physically scraped. The T H F was distilled before use to remove the inhibitor. The reactor and contents were sonicated to aid the dissolution and/or dispersion of the products in THF. This was repeated until the THF solution was clear, and the reactor tare weights before and after reaction agreed to within fO.O1 g. The total volume of THF so accumulated (-300 mL) was evaporated in a hood to a volume of about 50 mL and transferred to a tared 250-mL centrifuge bottle. Additional T H F was added to bring each volume up to 150 mL, and the bottles were centrifuged a t 3500 rpm for 20 min. The T H F solution was then decanted, and an additional 150 mL of T H F was added to the residue in each bottle with stirring. This centrifugation/decantation/solvent addition sequence was repeated until the initially opaque extract was amber and opalescent. Experience indicated that such a process was necessary twice in the higher conversion runs and up to five times with lower conversions. The residues (THF insolubles) were then dried overnight in their bottles in a vacuum oven at 110 "C under an aspiratormaintained vacuum. The oven was purged with nitrogen until the temperature was below 60 OC, and the bottles with residues were allowed to cool in air and were then weighed. The percent conversion was equal to w t of T H F insoluble residue - w t of ash 100 x 100 wt of maf coal The reproducibility was usually within 1%.
-
(12)Anderson, R. R.; Bockrath, B. C.Fuel 1984, 63, 329.
Mikita et al.
536 Energy & Fuels, Vol. 2, No. 4, 1988
run no. HD3-5 HD3-4 HD3-3 HD3-2 HD3-1 HD4-2 13, HD2-3, HD2-4 HD10-3, HD10-4 HD10-1, HD10-2 HD12-3, HD12-4 HD12-1, HD12-2 HD15-1, HD15-2 HD29-4 HD29-3 HD19-3, HD19-4 HD18-3, HD18-4 HD29-2 HD29-1
Table XI. Liquefaction of Illinois No. 6 (River King Mine) Bituminous Coal time at vehicle H2 pressure temp, (cold), psig temp, OC min amt.,g type gofH,O convn; % 800 370 60 1.62 LUM 0 55 800 370 60 2.00 LUM 0 67 1.29 LUM 1.70 61 800 370 60 800 370 60 1.60 LUM 1.70 80 370 60 2.00 LUM 1.70 79 800 800 (CO) 370 60 1.60 LUM 1.72 82 1185 427 60 1.98 LUM 1.70 86, 87 1200 385 40 1.50 MEN 1.70 77, 79 1200 385 40 1.50 LUM 1.70 82,84 385 40 1.50 SRC-I1 1.70 85, 87 1200 1200 385 40 1.50 TET 1.70 88, 90 1.50 DHA 1.70 91, 93 1200 385 40 385 30 0 ... 0 25 1200 385 30 0 3.4 52 1200 385 30 1.50 SRC-I1 0 72, 74 1200 1200 385 30 1.50 SRC-I1 1.7 86, 88 1200 385 30 3.4 CDOD ... 67 385 30 1.7 CDOD 1.7 72 1200
notes lummus vehicle
1-methylnaphthalene SRC distillate tetralin 9,lO-dihydroanthracene
...
cyclododecane cyclododecane
Based on THF extractions; coal charge in each case was 4.0 g. Table 111. Catalytic Experiments and Associated Base Cases in Water-Assisted Liquefaction of Illinois No. 6 Coal" expt. no. vehicle g of water ppm of Mo catalystb precursor/application' THF convn, % 6C SRC I1 1.7 ... 86,88 1.7 lo00 6D SRC I1 aqueous soln MoS4'90, 92 1.7 lo00 6E SRC I1 NH4 molybdate/soln slurry 89,91 1.7 (THF) 2000 6F SRC I1 90 Moz(OAc)4/solnslurry ... ... 3.4 6A surfact 67 1000 3.4 MoSt-/surfact soln ... 6G 86,90 1000 ... 3.4 6H NH4 molybdate/surfact soln 85,87 ... 1000 3.4 61 Most- impreg coal then water 91 1000 3.4 M ~ S ~ ~ - / s u r f soln act ... 6J 89, 91 ... 1000 6K NH4 molybdate impreg coal 76, 78 ... 3.4 1000 NH4 molybdate/surfact soln HD30-1 87 3.4 250 ... 80 HD30-2 NH4 molybdate/surfact soln 100 ... 3.4 73 HD30-3 NH4 molybdate/surfact soln ... 3.4 1000 HD30-4 NH4 molybdate/aqueous soln 84
...
'Reaction conditions: 4.00 g of River King coal; 385 "C; 30 min at temperature; 1200 psig (cold) of hydrogen, using 1.50 g of SRC-I1 distillate where indicated. bppm metal on coal. See Experimental Section for details. When the use of a surfactant is indicated, aerosol OT was employed at -200 ppm in water. The THF solutions obtained from the above procedure were rotoevaporated under an aspirator vacuum in tared, 250-mL round-bottom flasks. When no more solvent was seen to condense, the flasks were dried as before and the weight of THF-soluble products were thus obtained. In some mea,cyclohexane solubles were isolated by using a similar procedure. With the aid of sonication, the THF extracts were transferred to 250-mL centrifuge tubes with 25.0 1mL of hot THF. T o these solutions, 125 mL of cyclohexane was added and the centrifugation process employed, followed by a second extraction with an additional 150 mL of pure cyclohexane. The extracts were recovered by rotoevaporation of the solutions and drying in the vacuum oven a t 110 "C overnight. For a typical run, the gas analysis showed COz and H2S were the major gases produced, along with much lesser amounts of methane, ethane, and propane. The yields for one run were 4.2 wt % of CO, and 0.3 wt % each of methane, ethane, and propane on a daf coal basis. In experiments 6E and 6 F of Table 111, where a coal/water/ catalyst slurry was used, the procedure was as follows. Catalyst was added to 8.5 mL of a solution of 200 ppm of Aerosol OT surfactant (American Cyanamid) in water, and the mixture was slowly added to 10 g of coal in a round-bottom flask,with physical mixing, followed by immersion of the flask in a sonic bath for 5 min, followed by rotation of the flask on a rotoevaporator at atmospheric pressure while it was submerged in an ice bath for 15 min. This slurry did not appear to settle with time. The slurry was mixed by hand again just prior to transfer of 7.4 g to the microautoclave. In experiment 6D, an aqueous solution of catalyst was added to the coal in the autoclave, and in experiments 6G, 6H, and 6J,a solution of catalyst containing 200 ppm Aerosol OT
was added. In experiment 6K, a slurry made as in experiments 6E and 6 F above was dried in a vacuum oven before use. In experiment 61, such a dried slurry was added to the autoclave with distilled water.
Results and Discussion Vehicle Effects. Table I1 shows the results of runs made to measure conversion of Illinois No. 6 (River King Mine) bituminous coal under a variety of conditions selected to reveal effects of water addition. Since this work was directed toward replacing or reducing the amount of organic recycle vehicle normally employed, all of the experiments reported here were conducted with the relatively low vehicle-to-coal ratios of 0.5 or less. Experiments with higher ratios showed no better conversions. As m a y be seen, without addition of water, conversions for these high-coal-content slurries are q u i t e low at the very moderate reaction temperatures employed. The addition of 0.42 parts of water based on coal ( t h e a m o u n t contained i n a 70/30 coal/water slurry) increases conversion significantly. Evidently, reasonable conversions can be obtained with t h i s minimal amount of water provided the loading of recycle vehicle is not too low, that is, 1.6 g or above i n the case of experiments HD3-1, HD3-2, and HD3-3. We also note that under these mild, noncatalytic conditions, the nature of the reducing gas had little effect, since in one experiment (HD4-2),the substitution of carbon monoxide for hydrogen had little effect on the conversion. The use
Energy & Fuels, Vol. 2, No. 4, 1988 537
Liquefaction of Illinois Bituminous Coal
of the more severe conditions of 1200 psi of hydrogen (cold) and 427 "C produced a small increase in conversion as expected (experiments 13 and HD2-3). However, variables other than reaction severity may have more significant effects, since still higher conversions were obtained at 385 "C by an appropriate choice of the organic vehicle, as discussed later. A second series of experiments was run under conditions chosen to reveal the effect of the nature of the organic vehicle used in noncatalytic aqueous liquefaction. In a general way, conversions increased moderately as the organic vehicle was changed from one usually considered to be a poor hydrogen donor (1-methylnaphthalene) to vehicles considered to be better hydrogen donors (tetralin and 9,lO-dihydroanthracene). Thus, the usual concepts of hydrogen-donor liquefaction chemistry may also be applied usefully to this series of less conventional liquefaction systems. If good conversion in a noncatalytic system requires a minimal amount of an organic vehicle and also benefits from the addition of some water, it is reasonable to ask if the combination of organic vehicle and water is superior to either used separately. The final set of experiments in Table I1 indicates that this is the case. Conversion was poor without either (experiment HD29-4), much improved with water alone (experiment HD29-3), and still better with SRC-I1 distillate vehicle alone (experiment HD19-34). A combination of SRC-I1 distillate and water (experiments HD18-3-4) produced a conversion better than either used separately. Cyclododecane is a high-boiling hydrocarbon expected to be a poor hydrogen donor. When it was used as a nondonor vehicle, the conversion fell between the value for water and that for SRC-I1 distillate used in a lesser amount. When used in combination with SRC-I1distillate, the conversion was not improved over that of SRC-I1 distillate alone. This stands in contrast to the result obtained for the combination of SRC-I1 distillate plus water. This different behavior indicates that addition of the second component to SRC-I1distillate is not acting simply to dilute the relatively small amount of the recycle vehicle used in these experiments. Catalyst Effects. Table I11 contains data for the catalytic liquefaction of coal in the presence of water. When various forms of molybdenum are added to the coal, conversions are substantially increased. Clearly, high temperatures are not required for these molybdenum catalysts to influence liquefaction. More important, with catalyst present, the organic vehicle may be totally replaced by water without loss in conversion. As was seen in Table 11, this result was not obtained in the absence of catalyst. The important function of water is also underscored by the rather low conversion obtained with added catalyst but with no vehicle of any kind, i.e., a dry hydrogenation (experiment 6K). Conversion increases continually with catalyst loading up to at least 1000 ppm of molybdenum on coal (Experiments 6A, HD30-1, HD30-2, HD30-3). Of the several methods used for application of the catalyst, none seemed to give notably superior results insofar as THF conversion values are concerned. However, there are indications now under study that the yield of lighter products may change significantly according to the method of catalyst application. The surfactant, Aerosol OT, used to assist the preparation of coal/water/catalyst feed slurries, had a small beneficial effect on conversion in the presence of catalyst (experiment HD30-4 vs experiment HD30-1). At this
Table IV. Elemental Analyses and Molecular Weights of HD19-1and HD19-2Product Fractionsn cyclohexane THF THF solubles solublesc insolublesc elem anaLb C 82.9 81.8 24.5 H 7.31 5.8, 1.44 0 7.1 7.3 7.5 N 1.2 1.9 0.54 S 1.8 1.9 6.9 66.2 a_sh (750 OF), wt % MIl (570)d (1300)'
'Results are in wt % as reported by Huffman Laboratories, Wheatridge, CO. bAverage of two repeat microanalyses. cDried to constant weight at 30 "C under vacuum before analysis. VPO in benzene at 45 O C . Not completely soluble. 'VF'O in pyridine at 90 OC. Table V. Comparison of H 2 0 and D20as Vehicles" exDt no. vehicle time, min solvent c o n m b % 4A HzOc 20 tetralin 68 DzOC 20 tetralin 70 4B 5 SRC I1 55 HD 31-1, HD31-2 Hz0 5 SRCII 55 HD 31-3, HD31-4 DzO
Reaction Conditions: 4.00 g of River King Coal, 1.70 g of HzO or DzO; 1.50 g of solvent; 1200 psig of Hz (cold); 385 OC; 20 min at temperature. bOn a daf basis; average of duplicate runs; reproducibility *l wt %. 2 mmol of CszC08added via the water.
point, one can only speculate that the surfadant improves the wetting of the coal during impregnation with catalyst, presumably leading to better catalyst dispersion. In one case (6F), a low-boiling ether, THF, was substituted for water. The conversion was unexpectedly high, albeit the catalyst loading was twice that used with water. Tetrahydrofuran is quite effective in swelling River King coal at room temperature. The connection between this property and the liquefaction result is presently unclear, but investigation of a series of low molecular weight vehicles may shed more light on the structural requirements for superior performance. These and related experiments are in progress. Product Analysis. The data for analyses of the THF solubles isolated from experiments HD19-1 and HD19-2 in which 1000 ppm of Mo was used as catalyst in conjunction with surfactant are shown in Table IV. Except for the higher-than-usual oxygen content, these are typical analyses for a coal liquefaction product. The infrared spectrum of the cyclohexane solubles obtained from a dilute CH2C12solution shows that about 2.1 wt % of oxygen is in phenolic OH and the remaining 5% is presumably in ethers. The carbon aromaticity of the THF solubles was determined by 13C-CP/MASNMR spectroscopy to be 0.72, which is virtually identical with that of the feed coal. The data for characterization of the products are in accord with the rather mild conditions used to bring about the conversions. Isotope Studies. The observation of kinetic isotope effects often provides important information about reaction mechanisms. For example, inverse isotope effects have been observed by R o d 3 when water was replaced with D20 in coal liquefaction in a somewhat different system using CO/water without a solvent. In this system, a formate intermediate is proposed as the reducing species. Hydride transfer from formate to coal operates in competition with the water gas shift reaction. The inverse (13) Ross, D.S.;Green, T.K.; Mansani,R.;Hum, G. P. Energy Fuels 1987, 1 , 281.
Mikita et al.
538 Energy & Fuels, Vol. 2, No. 4, 1988 ~~
expt no. 5A 5B 5c 5D
Table VI. Aaueous Liauefaction without Molecular Hydrogen" est reacn water density, vehicle solventb catalystc pressure: psi g/cm3 DzO (22 mmol) tetralin KOH (8 mmol) 4350 0.110 HzO (22 mmol) tetralin KOH (8 mmol) 4350 0.097 HzO (9 mmol) tetralin 1790 0.043 HzO (9 mmol) SRC I1 1790 0.043
THF convn: % 42 45 46 26
"Reaction Conditions: 4.00 g of River King Coal; 385 "C; 40 min at temperature. b1.50 g per reactor. 'As an aqueous solution. dAs a daf basis; average of duplicate runs; reproduccalculated bv the van der Waal's eouation. Does not include solvent contributions. ibility fl %.
isotope effect, in the argument of Ross,13 occurs because the formate reaction with water to form formic acid experiences a normal isotope effect and is therefore slowed in D20. Formic acid decomposes rapidly to C02 and Hz (D2). Consequently, in D20 the steady-state concentration of active formate is increased relative to that in HzO. A search for kinetic isotope effect was conducted in three sets of experiments, each comparing H,O with DzO, in order to determine if water has a kinetic role in the present case. In the first set, tetralin was employed as the solvent, and a hydrogen charge was utilized. In an attempt to enhance the ionic character of these reactions, 2 mmol of CszC03was added to the water. The Cs2C03was chosen because Cs has been reported as the most mobile of the group 1A metals in the gasification of coa1s,14 and we wished to circumvent, as far as possible, any anomalies from selected ion exchange. The conversions obtained with HzO and D20, 68-70 wt 5% , were identical within experimental error (see Table V). Clearly, under these conditions, no isotope effect is evident. The second set of experiments was done at much shorter reaction times (5 min) to determine if the kinetic isotope effects would be more pronounced a t lower conversions. A less potent hydrogen donor, SRC-11, was used to accentuate any chemistry associated with water. In this case, no base was added as a catalyst so that hydrolysis reactions would not be masked. Again, no isotope effect was observed. Finally, four experiments without molecular hydrogen were also conducted, as illustrated in Table VI. Comparison of H20 and DzOwith added KOH as a catalyst and tetralin as the solvent resulted in slightly lower conversions for the DzO than for the H 2 0 experiments. Most significantly, the conversionswere considerably lower than those with less water but with molecular hydrogen (see Table V). Also, removal of the KOH catalyst and reduction of water by a factor of 2 resulted in no appreciable change in conversion (compare experiments 5B and 5C). Clearly, molecular hydrogen plays a dominant role even in the presence of a base catalyst. A change in solvent from tetralin to SRC-11, however, gave a dramatic decrease in conversion. Such an effect may be easily attributed to a lower number of hydrogen donors in SRC-I1 when compared to tetralin. The sig(14) Mims, C.; Past, J. Fuel 1980, 62, 176.
nificant role of such donors in uncatalyzed water-assisted liquefaction is clearly evident from these experiments. Taken together, these results provide no evidence that breaking the O-H bond in water is a factor that governs the rate of production of soluble products. Also, the data reaffirm the central role of either of the two hydrogen sources that are conventionally used, molecular hydrogen and organic hydrogen donor solvents.
Conclusions The results obtained so far form a basis for an encouraging outlook for the use of water or other nondonor vehicles as liquefaction media in fundamental studies of coal conversion. High coal conversions were obtained at modest temperatures with little or no organic recycle vehicle. Thus, it is possible and feasible to use water as a substitute liquefaction medium. No advantage to using water at the cost of reducing hydrogen partial pressure was found when the total pressure in the reactor was fixed by economic consideration^.'^ Still, a line of investigation is now open whereby important questions regarding the mechanism of liquefaction may be addressed. Since no organic vehicle need be used, the coal-derived products of liquefaction may be analyzed without interference. At the same time, the presence of a fluid medium provides better heat transfer and better dispersion of the reactants than possible with dry hydrogenations. In fact, coal conversion in water is greater than with dry hydrogenation under otherwise comparable conditions. Work on the determination of how the course of liquefaction is affected by the chemical and physical properties of water or other nondonor solvents in conjunction with the use of dispersed catalysts at mild liquefaction temperature is under way.
Acknowledgment. M.A.M. was a Faculty Research Participant for the summer of 1984 and was appointed to the Pittsburgh Energy Technology Center (PETC) by Oak Ridge Associated Universities; M.A.M. also wishes to acknowledge additional financial support from DOE (Grant DE-FG22-85PC81544). We wish to thank the personnel from the PETC Division of Coal Science for careful analyses of the various materials and productb. Encouragement and helpful suggestions from B. D. Blaustein are also acknowledged. (16)Ruether, J. A.; Mima, J. A.; Kornosky, R. M.; Ha, B. C. Energy Fuels 1987, I , 198.
