Energy & Fuels 1998, 12, 607-611
607
Liquefaction of Wyodak Coal with Phosphomolybdic Acid Belma Demirel and Edwin N. Givens* University of Kentucky Center for Applied Energy Research, 2540 Research Park Drive, Lexington, Kentucky 40511-8410 Received September 30, 1997
Phosphomolybdic acid (PMA) has been found to be a very effective catalyst precursor for direct liquefaction of Wyodak coal. A concentration of 300 mg Mo/kg maf coal of PMA imparts greater activity when impregnated onto the coal compared to adding the crystalline solid directly to the reaction mixture. Significant improvement in the reaction occurred only when H2S was added to the reaction mixture, suggesting that the Keggin anion does not act as the catalyst. Impregnated PMA is at least as active as organically soluble molybdenum di(2-ethylhexyl)phosphorodithioate or oxothiomolybdenum N,N-dibutyldithiocarbamate. The method of impregnating the PMA from aqueous solution onto the coal did not have any significant influence on THF solubilization or resid conversion. It was also found that neither THF solubilization nor conversion to distillate was affected by impregnating the Mo precursor onto only 10% of the feed coal. However, if the PMA is impregnated onto a carbon carrier, which is then mixed with coal, very poor THF solubilization occurs.
Introduction A number of Mo-containing materials which are soluble in the liquefaction process solvent are known to give active catalysts for conversion of coal because of their high dispersion in the reacting phase. Unfortunately, most of these materials are quite expensive which would be prohibitive in a large-scale application. A number of different coal-impregnated Mo precursors have been found to provide active catalysts for direct liquefaction of coal,1 presumably because the precursor forms a highly dispersed catalyst in the reaction phase. By using readily available, low cost water-soluble molybdenum salts to impregnate coal, the cost associated with preparing organically soluble precursors can be avoided. Through judicious selection of the metal salt, catalyst cost can be reduced to a level that is only marginally greater than for the cost of molybdenum trioxide produced in the refining of Mo-rich ores. Phosphomolybdic acid (PMA) is such a water-soluble form of Mo that can be prepared by a very simple procedure starting with MoO3 and phosphoric acid.2 PMA is an especially unique form of Mo that has already found extensive use as a catalyst in a number of other processes.3,4 Oxidation catalysis by PMA has been attributed to the unique combination of electron transfer and acidic properties of the Keggin anion, i.e., (PMo12O40)3-. There has been considerable interest in the use of catalysts prepared from PMA for conversion (1) Weller, S. W. Catalysis in the Liquid-Phase Hydrogenation of Coal and Tar. In Catalysis; Emmett, P. H., Ed.; Reinhold: New York, 1956; Vol. 4, p 513. Hawk, C. O.; Hiteshue, R. W. Hydrogenation of Coal in the Batch Autoclave. U. S. Burea of Mines Bulletin 622, 1965. (2) Tsigdinos, G. A. Ind. Eng. Chem., Prod. Res. Dev. 1974, 13 (4), 267-274. (3) Goodenough, J. B. Solid State Ionics 1988, 26, 87-100. (4) Misono, M. Appl. Catal. 1990, 64, 1-30.
of heavy hydrocarbons and carbonaceous-containing materials. Though most of the work relates to conversion of petroleum-related heavy hydrocarbons, there are in the patent literature examples that describe the use of PMA in conversion of coal. In one of the earliest patents, Gleim and Gatsis5 claimed a process for hydrorefining a petroleum oil at 400 °C and 10 MPa. In this case PMA was added to the reaction mixture as an isoamyl alcohol solution, which presumably forms a colloidal dispersion when the isoamyl alcohol is removed by distillation at 130 °C.6 In the hydroconversion of Wyodak coal, a higher oil yield and lower coke make were reported for PMA relative to molybdenum naphthenate, both of which were claimed to be soluble in the heavy distillate solvent.7 In another case, addition of phenolic solutions of PMA to coal slurries was reported to give higher liquid yields.8 Several cases claim preparation of catalyst concentrates by mixing either aqueous or isopropyl alcohol solutions of PMA with oil and heating at 385 °C for 30 min in 5% H2S in H2.9-12 (5) Gleim, W. K. T.; Gatsis, J. G. Hydrorefining crude oils with colloidally dispersed catalysts. U.S. Patent 3161585, Dec. 15, 1964. (6) Gatsis, J. G. Hydrorefining of petroleum crude oil. U.S. Patent 3249530, May 3, 1966. (7) Aldridge, C. L.; Bearden, R. Hydroconversion of coal in a hydrogen donor solvent with an oil-soluble catalyst. U.S. Patent 4077867, Mar. 7, 1978. (8) Aldridge, C. L.; Bearden, R. Coal liquefaction process. U.S. Patent 4369106, Jan. 18, 1983. (9) Bearden, R.; Aldridge, C. L.; Mayer, F. X.; Taylor, J. H.; Lewis, W. E.Method of preparing a hydroconversion sulfided molybdenum catalyst concentrate. U.S. Patent 4740489, Apr. 26, 1988. (10) Bearden, R.; Aldridge, C. L.; Mayer, F. X.; Taylor, J. H.; Lewis, W. E. Hydroconversion process using a sulfided molybdenum catalyst concentrate. U.S. Patent 4740295, Apr. 26, 1988. (11) Bearden, R.; Aldridge, C. L. Hydrocracking with aqueous phosphomolybdic acid. U.S. Patent 4637871, Jan. 20, 1987. (12) Bearden, R.; Aldridge, C. L.Hydrocracking with phosphomolybdic acid and phosphoric acid. U.S. Patent 4637870, Jan. 20, 1987.
S0887-0624(97)00189-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/03/1998
608 Energy & Fuels, Vol. 12, No. 3, 1998
Demirel and Givens
Table 1. Black Thunder Coal Analysis proximate anal (wt %) moisture ash volatile matter fixed carbon
8.9 6.0 43.48 50.52
ultimate anal (wt %) carbon hydrogen nitrogen sulfur oxygen (diff) ash ash, SO3-free
70.12 5.11 0.99 0.35 17.24 6.19 5.69
Addition of phosphoric acid or hydrogen halides to PMA has been reported to improve catalyst activity.13,14 Combinations of MoO3 and PMA have also been reported to give better resid conversion than the individual compounds.15-17 In applying PMA to coal liquefaction, the reaction conditions differ significantly from oxidation type applications, which have been extensively studied.18 In liquefaction, not only does the reaction take place in a reducing atmosphere but the reaction temperature is higher and sulfur is also present. Also, the presence of a sizable concentration of nitrogen bases in these reaction systems may influence the activity of the PMA through neutralization of the acidic function. Because of the unique stability of PMA at higher temperatures, the question arises as to whether the Keggin ion will continue to exist during the liquefaction reaction. In this paper the results are presented of a study in which the liquefaction activity of PMA-derived catalysts were evaluated relative to other Mo compounds that have been applied to this application. Low-rank Wyodak coal was impregnated with aqueous solutions of PMA at a level of 300 mg Mo/kg dry coal and tested in a resid-containing Wyodak coal derived solvent. The thermal behavior of PMA and its reaction with H2 and H2S at elevated temperatures is discussed to provide insight into the transformations that may be occurring in the reaction system. Experimental Section Tetrahydrofuran was obtained from Burdick & Jackson; phosphomolybdic acid, activated carbon and molybdenum trioxide were obtained from Aldrich; hydrogen sulfide (99.5% purity) and hydrogen (UHP 6000#) were obtained from Air Products and Chemicals, Inc. Molyvan A, N,N-dibutyldithiocarbamate of oxothiomolybdenum,19 and Molyvan L, molybdenum di(2-ethylhexyl)phosphorodithioate in a petroleum process oil (Mo, 8.1%; P, 6.4%), were supplied by R. T. Vanderbilt Co. Wyodak coal, obtained from the Black Thunder (BT) Mine in Wright, Wyoming, was ground (90% -200 mesh), riffled, and stored under nitrogen at 4 °C. Proximate and ultimate analyses of the coal are presented in Table 1. Heavy distillate (565 °C, Wilsonville Vessel Number (13) Aldridge, C. L.; Bearden, R. Hydroconversion process. U.S. Patent 4196072, Apr. 1, 1980. (14) Bearden, R.; Baird, W. C.; Aldridge, C. L. Hydroconversion process. U.S. Patent 4424110, Jan. 3, 1984. (15) Gatsis, J. G. Catalyst for the hydroconversion of asphaltenecontaining hydrocarbonaceous charge stocks. U.S. Patent 5288681, Feb. 22, 1994. (16) Gatsis, J. G. Catalyst for the hydroconversion of asphaltenecontaining hydrocarbonaceous charge stocks. U.S. Patent 5474977, Dec. 12, 1995. (17) Gatsis, J. G. Method of preparing a catalyst for hydroconversion of asphaltene-containing hydrocarbonaceous charge stocks. U.S. Patent 5171727, Dec. 15, 1992. (18) Misono, M. Catal. Rev.sSci. Eng. 1987, 29, 269-321. (19) Moore, F. W.; Larson, M. L. Inorg. Chem. 1967, 6, 998.
V-130, Period a) used in this study were produced in run 258 at the Advanced Coal Liquefaction R&D Facility at Wilsonville, Alabama, when the plant was operating in a close-coupled configuration and feeding Wyodak coal from the Black Thunder mine.20 Metal-impregnated coals were prepared by mixing aqueous PMA solutions with BT coal containing 8.9 wt % moisture. Spiked coals were prepared by impregnating a portion of the coal (10%) with PMA solution and further mixing with fresh coal thereby diluting the concentration of Mo in the mix. In every case the concentration of PMA in the final coal mix was equivalent to 300 mg Mo/kg dry coal. The following are the methods that were used to prepare the individual metalimpregnated coals: (i) Coal I was prepared by slurrying BT coal with 3 times its weight of PMA containing solution for 2 h after which excess water was removed by evaporation at 50-60 °C. The cake was dried at 96 °C and 33 kPa overnight to remove essentially all of the moisture. (ii) Coal II was prepared by the method in (i) except the cake was dried at 80-90 °C under vacuum at 17 kPa to give a final moisture content of 1.77 wt %. (iii) Coal III was prepared by slowly adding a PMA-containing solution to BT coal (0.45 g solution/g dry coal) and then drying at 96 °C and 30 kPa overnight to a moisture level of 3.9 wt %. (iv) Coal IV was prepared by a procedure similar to (iii) except that PMA was dissolved into a smaller amount of water, i.e., 0.03 g/g dry coal. The impregnated coal was not further dried before use and contained 11.3 wt % moisture. (v) Coal I-S was prepared by the procedure described in (i) except that the PMA in the solution was equivalent to 3000 mg Mo/kg dry coal. The resulting impregnated coal was then mixed with 9 times its weight of BT coal (dry basis) giving a final moisture content of 8.0 wt % on dry coal. (vi) Coal II-S was prepared by the procedure described in (ii) except that the solution contained PMA equivalent to 3000 mg Mo/kg dry coal. Unlike in (ii), this coal was dried to a final moisture content of 12.79 wt %. After combining impregnated coal (dry basis) with 9 times its weight of BT coal (dry basis) the final moisture content was 9.3 wt %. (vii) Coal III-S was prepared by the procedure described in (iii) except that the impregnating solution contained PMA equivalent to 3000 mg Mo/kg dry coal. The impregnated coal (dry basis) was then mixed with 9 times its weight of BT coal (dry basis) to give a final moisture content of 9 wt %. (viii) Coal IV-S was prepared by the procedure described in (iv) except that the impregnating solution contained PMA equivalent to a Mo loading of 3000 mg Mo/kg dry coal. The impregnated coal (dry basis) was then mixed with 9 times its weight of BT coal (dry basis) giving a final moisture content of 11.3 wt %. (ix) An impregnated carbon (carbon 10) was prepared by mixing activated carbon, having a BET surface area of 790 m2/g, with 3 times its weight of a solution containing PMA equivalent to 3000 mg Mo/kg activated carbon at ambient temperature for 2 h. Excess water was removed by evaporation at 50-60 °C and the resulting cake was dried at 96 °C and 33 kPa. Dried cake was mixed with 10 times its weight of BT coal (dry basis). (x) An impregnated carbon (carbon 5) was prepared by impregnating the activated carbon described in (ix) by the method described in (ix) with a solution containing PMA equivalent to 6000 mg Mo/kg dry activated carbon. The dried impregnated carbon was mixed with 20 times its weight of BT coal. (20) Southern Electric International, Inc. Run 258 with subbituminous coal. U.S. Department of Energy Report No. DOE/PC/50041-130, Wilsonville, Al, 1991.
Liquefaction of Wyodak Coal
Energy & Fuels, Vol. 12, No. 3, 1998 609
Reactions were conducted at 440 °C for 30 min in 50 cm3 microautoclaves that were equipped with a thermocouple and pressure transducer for monitoring temperature and pressure, respectively, during the reaction. In a typical experiment, 1.75 g of heavy distillate, 2.8 g of ash-free resid, and 2.45 g of moisture-free equivalent metal impregnated coal were added to the reactor. Other experiments were run in which the precursors were added directly into the reactor. These precursors were either particulate materials (MoO3, PMA), an organic solution of the Mo precursor (Molyvan L), or an organic soluble material (Molyvan A).19 MoO3 and PMA, especially at lower temperatures, are insoluble in the process solvent. After the reactor was loaded, it was purged with N2, pressurized with a mixture of H2 and H2S (3% H2S in H2), submerged in a fluidized sand bath, and continuously agitated at a rate of 400 cycles per minute while monitoring temperature and pressure. At the end of the reaction period, the reactor was removed from the sand bath and quenched to ambient temperature and the gaseous products were collected and analyzed by gas chromatography. The solid and liquid products were removed from the reactor using THF and the mixture was extracted in a Soxhlet extraction apparatus for 18 h. The THF-insoluble material was dried (80 °C at 17 kPa) and weighed. Vacuum distillations were performed using a modified ASTM D-1160 procedure starting with the residue from THF extraction of the total reactor effluent.21 Cut points were adjusted in several cases based upon a GC simulated distillation procedure that had been developed at the CAER. Material balances are based upon a forced ash balance with any loss and error being accounted for in the distillate product. Product distributions using the distillation method are calculated by subtracting the weights of gases, residue, ash, and insoluble organic matter (IOM) in the product from the weight of each in the feed. Resid conversion was calculated on a moisture and ash free (maf) basis as follows:
resid conv ) 100 ×
[coal + resid]feed-[IOM + resid]product [coal]feed
Coal conversion equals 100 minus the yield of IOM.
Results and Discussion Liquefaction typically takes place at temperatures between approximately 400 and 450 °C. It is at these temperatures that the Keggin ion is known to decompose, although there are cationic forms that are stable at higher temperatures.22 When PMA is heated in an inert atmosphere (He) at temperatures up to about 150 °C, it first loses approximately 15% of its weight via an endothermic process.2 This corresponds to the loss of crystalline water or water of hydration. Between 150 and 350 °C, there is only a slight additional loss in weight. However, heating to ∼400 °C causes a significant loss in weight that corresponds to elimination of constitutional water and results in destruction of the Keggin ion and formation of P2O5 and MoO3. In H2, the thermal behavior is nearly the same as in He except for the different form of the decomposed structure. Tsigdinos observed in N2 that only MoO3 was formed, whereas Yong et al.22 observed in H2 that both MoO2 and free metal are formed. Prolonged heating in H2 at 300 °C does not destroy the Keggin anion but results in (21) Standard Test Method for Distillation of Petroleum Products at Reduced Pressures, Method No. D-1160-87. In Annual Book of ASTM Standards; Am. Soc. Testing Materials: Philadelphia, PA, 1990; Vol 05.01, p 419. (22) Yong, W. J.; Quan, X. X.; Zheng, J. T. Thermochim. Acta 1987, 111, 325-333.
broadening of the Mo-O bonds in the IR.23 Yong et al. observed that part of the Mo6+ is reduced to Mo5+ atoms in this temperature range with electron transfer and formation of H+.22 The essential features of the Keggin structure remain. If temperature is further increased, the H+ ions combine with structural oxygen forming water leading to destruction of the Keggin ion. Shokhireva et al. reported that treating PMA with H2S at temperatures up to 300 °C causes structural changes but does not destroy the Keggin anion.23 However, at 400 °C, the anion is ultimately destroyed producing MoS2 and MoOx. PMA has received very little attention as a precursor in coal liquefaction relative to ammonium molybdate or several other organic soluble Mo-containing precursors. In this study, we have evaluated PMA for this application using a liquefaction test protocol that had been developed earlier for evaluating other metal-containing precursors.24 In this test, a solvent is used that contains a sizable amount of solids-free resid, which resembles the composition of solvents that would actually exist in a plant recycle operation. Although solids comprising unconverted coal and mineral matter are also typically in these streams, they were intentionally not included in the test solvent in order to eliminate the complications associated with the presence of recirculated catalyst.24 The solvent that was used comprises a 565 °Cdistillate and a 565 °C+ solids-free resid, both of which were produced at Wilsonville in run 258. The solidsfree resid component was produced in a ROSE-SR unit by deashing the solids-containing process stream.25 In our test experiments, distillate, deashed resid, and feed coal were blended to give a resid/maf coal ratio that was approximately the same as that used at Wilsonville. When a sulfiding agent was added, H2S was introduced as 3 vol % H2S in H2 (4 wt % S on dry coal). A precursor loading of approximately 300 mg Mo/kg dry coal was used throughout. A series of experiments was performed to determine if sulfur addition provided any beneficial effect. If the catalytic activity of PMA in liquefaction is due to the Keggin ion, as observed in a number of other reactions,18 the formation of Mo-S structures would not be required. Adding H2S to runs in which crystalline PMA was added directly to the reaction mixture resulted in a significant increase in both THF and resid conversions. By adding H2S, THF conversion increased from 66.9% to 80.7% and resid conversion increased from 29.3% to 42.7% on maf coal (see Table 2). In the absence of H2S, the magnitude of the effect of PMA in the reaction mixture was negligible. These results suggest that the Keggin ion is not the active catalyst. It may be converted by addition of sulfur to the anion or destroyed to produce molybdenum sulfide. We observed also that Molyvan L and Molyvan A are both considerably more active in the presence of H2S. Because of these results, H2S was routinely added to the reaction mixture for all of the PMA runs. (23) Shokhireva, T. K.; Yurieva, T. M.; Altynnikov, A. A.; Anufrienko, V. F.; Plyasova, L. M.; Litvak, G. S. React. Kinet. Catal. Lett. 1992, 47 (2), 177-185. (24) Rantell, T., Anderson, R. K. and Givens, E. N. Prepr. Pap.sAm. Chem. Soc., Div. Fuel Chem. 1996, 41 (3), 993. (25) Adams, R. M.; Kneve., A. H.; Rhodes, D. E. Chem. Eng. Prog. 1979, 75 (6), 44-48.
610 Energy & Fuels, Vol. 12, No. 3, 1998
Demirel and Givens
Table 2. Activity of Precursors Added Directly to Reaction Mixture precursor none PMA PMA Molyvan L Molyvan L Molyvan A Molyvan A
Mo conc, mg/kg vol % H2S THF resid conversion, in H2 conv wt % maf coal dry coal none 476 325 344 294 295 330
0 0 3 0 3 0 3
66.3 66.9 80.7 77.5 85.4 76.7 91.2
28.1 29.3 42.7 34.1 57.0 35.1 60.7
In their work using PMA as a catalyst for coal liquefaction, Aldridge and Bearden claimed that PMA was an effective catalyst precursor for conversion of Wyodak coal because of its oil solubility, which resulted in an increase in catalyst dispersion in the reaction solvent.7 If indeed PMA is soluble, this would provide a very simple route by which to introduce a highly dispersed, relatively inexpensive form of Mo into the reaction system. Our attempt to dissolve 0.1 wt % PMA in process distillate by heating for an extended period at 150 °C was not successful, as observed by the continued presence of granular PMA in the reaction media. This observation agrees with those of Tsigdinos who reported that PMA is insoluble in common hydrocarbon solvents.2 If, however, PMA does dissolve in process solvent, especially at higher temperatures, its activity would be expected to be significantly greater than for MoO3, which is a crystalline solid and not soluble in organic media. The results that were obtained from runs made with crystalline PMA and MoO3 show that the THF solubilization activity for crystalline PMA was slightly higher than for MoO3 while resid conversions were essentially the same (see Table 2). The conversions achieved with Molyvan L and Molyvan A, both of which are soluble in heavy hydrocarbon oil and give highly dispersed forms of Mo, are considerably greater than achieved with crystalline PMA. Although crystalline PMA is active, its level of activity is considerably less than achieved with a highly dispersed form of Mo. An alternative way of introducing PMA into the reaction system, in lieu of adding as a crystalline material, is to impregnate the acid onto coal. In all of the previous work with this material, it had been either added directly to the reaction mixture as a crystalline material or dissolved in phenol or an alcohol and mixed with the process solvent. The approach that has been used in this study has been to impregnate the coal with an aqueous solution of PMA. Since the method of impregnating with aqueous salt solutions has been reported to affect the coal reaction, we investigated the effect of the amount of water used in impregnating the acid onto the coal. Previous work had shown that a uniform distribution of Mo on the coal surface was obtained when coal was impregnated with aqueous solutions of ammonium molybdate using an incipient wetness technique, i.e., 0.45 g water/g dry coal.26 In this study, coals were impregnated with PMA solutions using (i) a large excess of water, i.e., three volumes per volume of coal, (ii) an incipient wetness amount of water, i.e., 0.45 g water per g dry coal, and (iii) a smaller (26) Ni, H., Anderson, R. K., and Givens, E. N. Energy Fuels 1994, 8, 1316-1323.
Table 3. Activity of Coals Impregnated with PMA at 300 mg Mo/kg Dry Coal impregnated coal coal I coal II coal III coal IV all samples N ) 11
no. of moisture, runs wt % 1 3 5 2
none 1.8 3.9 11.3
THF conv
resid conversion, wt % maf coal
90.0 95.0 ( 0.7 91.1 ( 2.1 91.5 ( 1.4 92.3 ( 2.4
64.6 70.2 ( 4.9 61.1 ( 4.8 64.7 ( 0.9 64.5 ( 5.4
Table 4. Activity of 10% Spiked Coals spiked impregnated coal coal I-S coal II-S coal III-S coal IV-S all samples N ) 11
no. of moisture, runs wt % 1 3 4 3
8.0 9.3 8.0 11.3
THF conv
resid conversion, wt % maf coal
90.8 93.1 ( 1.9 89.1 ( 1.6 93.6 ( 2.8 91.6 ( 2.7
70.4 62.0 ( 2.9 57.9 ( 3.5 61.9 ( 1.8 61.2 ( 4.4
amount of water equivalent to only 3 wt % of the starting coal. As a further variation, the effect of metal maldistribution over the surface was also investigated by spiking the metal onto only 10% of the starting coal, which was then mixed with the remaining 90%. Previously, Cugini et al. had found that spiking the precursor onto a portion of the feed coal (10-20%) had no noticeable effect on liquefaction performance.27 Results from liquefaction of the PMA impregnated coals show that both THF solubilization and resid conversions were consistently higher than the corresponding values observed for crystalline PMA (see Table 3). Averaged THF conversions for these impregnated coals ranged between 91 and 95 wt % with a higher THF conversion observed for coal II, which was prepared by rotovaping off excess water down to a moisture content of 1.8 wt %. Resid conversions for this series ranged from 61 to 70 wt % with no significant differences between the values. Averaged resid conversion, ignoring the differences in method of preparation, was 64.5 ( 2.4 wt % maf coal (n ) 11). There is no question that impregnated coals produce significantly higher resid and THF conversions than obtained by adding crystalline PMA. In addition, the conversions for impregnated coals were at least equal to the conversions obtained for Molyvan L and Molyvan A (Table 2). Overall, the data show that impregnated PMA is an excellent catalyst precursor that is not significantly affected by the method used to impregnate the coal. The effect of metal maldistribution on the reactivity of impregnated coal was determined by liquefying the four spiked coals that were prepared. In each preparation PMA equivalent to 3000 mg Mo/kg dry coal was impregnated onto coal which was subsequently mixed with raw coal in a ratio of 1/9 (dry basis) to give spiked coals containing the equivalent of 300 mg Mo/kg dry coal. The results from testing these coals are shown in Table 4. THF conversions for the four differently prepared spiked coals range from 89 to 93% with the THF conversion for the coal prepared by rotovaping off excess water being higher than the others. Resid conversions, on the other hand, are narrowly grouped with an averaged conversion of 61.2 ( 4.4%. Overall, (27) Cugini, A. V.; Krastman, D.; Lett, R. G.; Ciocco, M. V.; Erinc, J. B. The Effect of Dispersed Catalysts on First Stage Coal Liquefaction. In Proceedings Pittsburgh Energy Technology Center Coal Liquefaction and Gas Conversion Contractor’s Review Conference, Pittsburgh, PA, Sept. 27-29, 1993; Vol. 1, p 485.
Liquefaction of Wyodak Coal
Energy & Fuels, Vol. 12, No. 3, 1998 611
Table 5. Activity of PMA-Impregnated Carbons at 300 mg Mo/kg Dry Coal spiked impregnated carbon
moisture, wt %
THF conv
resid conversion, wt % maf coal
carbon 5 carbon 10
none none
82.2 80.9
62.2 64.6
the data indicate that coals prepared by impregnating PMA onto only 10% of the feed coal give essentially the same THF and resid conversions as impregnating the PMA onto all of the coal. To what extent this maldistribution of the precursor can be carried is not known except that in the extreme, where the PMA is not impregnated onto the coal, a sharp drop in conversion activity occurs. The unique feature about coal as a catalyst support is that in the reaction the support disappears as it transforms into the liquid state. Presumably as this happens, the precursor becomes suspended within the gel-liquid state. At the same time, it may be decomposing or reacting with the H2 and H2S while remaining as a very small solid particle. Presumably, as these changes occur, the catalyst that is formed remains highly dispersed within the reaction media, thereby being more effective than if it had been deposited onto a solid surface. At the Mo levels that are used in these reactions, earlier work had shown that similar concentrations of Mo on supported catalysts had negligible effect.28 Presumably, this result would also apply when PMA is used as the source of Mo. In order to determine if this disappearing support mechanism is operable in this system, PMA was spiked onto an activated carbon at the same level as used in the spiked coal experiments discussed above. An activated carbon was impregnated with PMA to a level of 3000 and 6000 mg Mo/kg activated carbon and then mixed with BT coal to give final Mo concentrations equivalent to 300 mg Mo/kg dry coal. Liquefaction of these two samples, as shown in Table 5, indicates that THF solubilizations are significantly lower than observed for the coal impregnated samples. On the other hand, the resid conversion for the PMA impregnated carbons remained about the same as observed for the impregnated coals. This may imply that when the Mo is impregnated onto the activated carbon, the conversion of the resid in the reaction mixture and the newly created resid formed from the liquefying coal is more readily converted to distillate even though more of the reacting mass forms insoluble organic material. From a process standpoint, even though resid conversions remained high, the process would be untenable because of the abnormally large amount of unconverted coal which would have a negative impact on process economics. (28) University of Kentucky Center for Applied Energy Research. Advanced Direct Liquefaction Concepts for PETC Generic Units; Phase I Final Report. U.S. Department of Energy Report No. DOE/PC/9104055, Lexington, KY, 1995; page 3-1.
The decrease in THF conversion that resulted for the impregnated carbons suggests that the more intimate contact between the disintegrating coal particle and the precursor suspended within the liquefying mass, which is being transformed into an active catalytic form, either promotes the breakdown of the polymeric structure into more discrete, lower molecular weight fragments or serves to prevent retrogressive polymerization reactions between the fragments as they form. In the case where the precursor is originally deposited onto a surface that remains intact during the reaction, the likelihood of contact between the catalyst that forms on that support and the liquefying coal particle is considerably reduced since the catalyst will not likely become detached from the carbon support and diffuse through the reaction media to the extent achieved in the impregnation case. On the other hand, the continued high resid conversion suggests that cracking of the heavier dissolved fragments is equally effective where the solubilized molecule can diffuse to the supported Mo catalyst. These results show that coal solubilization and conversion to distillate appear to be independent reactions. Conclusions PMA is a water-soluble form of Mo that can be prepared by a very simple procedure starting with MoO3 and phosphoric acid. The anion has a well-ordered Keggin ion crystal structure, which is known to be stable in inert atmospheres, as well as in H2, at temperatures up to approximately 400 °C. Even in the presence of H2S, it is stable at temperatures up to 300 °C but is ultimately destroyed at 400 °C to form MoS2 and MoOx. Because of this stability at these temperatures, which approach liquefaction reaction temperatures, the rate at which PMA converts into an active catalyst form is uncertain. The catalyst generated from PMA in the presence of H2S is quite active for coal liquefaction since it increases both coal solubilization and resid conversion. The activity achieved by impregnating the PMA onto coal is considerably greater than by adding crystalline PMA to the reaction mixture. The level of conversion with impregnated coals is equivalent to or better than for either Molyvan L or Molyvan A, two organic soluble Mo materials. Conversions from spiking PMA onto only 10% of the feed coal were essentially the same as impregnating the PMA onto all of the feed coal. When the PMA was impregnated onto an activated carbon, THF solubilization decreased sharply making such an approach untenable from a process standpoint. Acknowledgment. The financial support of the Department of Energy under contract no. DOE AC2291PC91040 is gratefully acknowledged. EF9701899
