Energy & Fuels 1994,8, 25-30
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Design of Recoverable Catalysts for a Multistage Coal Liquefaction Process Isao Mochida,' Kinya Sakanishi, Ryuji Sakata, Katsuyuki Honda, and Tatsuya Umezawa Institute of Advanced Material Study, Kyushu University, Kasuga, Fukuoka 816, Japan Received July 12, 1993. Revised Manuscript Received October 28, 199P
A multistage liquefaction scheme consisting of coal pretreatment, solvent-mediated hydrogentransfer dissolution, and catalytic hydrocracking steps was proposed in order to achieve the highest oil yields with complete conversion of organic components in coal and to enable the recovery and repeated use of the catalyst with its least deactivation. The deashing pretreatment enhanced the depolymerization of coal macromolecules through the removal of bridging ion-exchangeable cations as well as the better contact of donor solvent with coal macromolecules. The highest oil plus asphaltene yield of ca. 75% was obtained through the two-step liquefaction of acid-treated Morwell coal by the reactions of noncatalytic hydrogen transfer (donor solvent: tetrahydrofluoranthene, solvent/coal = 1)at 430 OC-2 min in the first step and the following catalytic hydrogenation with a pyrite catalyst at 400 "C-20 min. The multistep liquefaction scheme is discussed in terms of the highest efficiency of utilization of solvent and catalyst at the respective step. Two types of recoverable catalysts were investigated to allow repeated use of the catalyst in the primary liquefaction. The first type was an acid-proof iron catalyst which was recoverable from a mixture with residual carbonates and chlorides that are soluble in acids. Such a catalyst was applicable to particular coals such as an Australian brown coal which is completely liquefied, leaving calcium and magnesium carbonates as the major residual minerals after the primary liquefaction. The second type was characterized by its sulfur-proof ferromagnetism for the recovery from the minerals and carbons by gradient magnetic field. Fe3A1 powder and carbon/ferrite composite catalysts were found to maintain their ferromagnetism after the sulfiding to be fairly active in the liquefaction of the brown coal. Procedures for activation, recovery, and repeated use of these recoverable catalysts were preliminarily examined.
Introduction
Coal liquefaction has been rather extensively investigated for more than several decades to provide clean liquid fuel from coal in order to meet the increasing demand expected in early next century. However, the cost of the liquid fuel is still too high to substitute the fuel from petroleum. Several breakthrough ideas are strongly desired to cut the cost currently estimated. The authors believe that the complete conversion of coal without organic residue and catalyst recovery for its repeated use is a target for research, since increasing the oil yield and reducing the waste are expected to cut the cost. The combined utilization of solvent and catalyst for primary coal liquefaction processes has been extensively investigated by many researchers in order to increase the distillate yield and improve the efficiency of hydrogen consumption.' German groups insisted that the liquefaction at high temperatures (- 500 "C) and high pressure (-300 atm) conditions can provide an excellent oil yield regardless of the solvent or catalyst species? while other groups such as NBCL, NEDOL, and PETC examined the effectiveness of solvent in the presence of catalyst under much milder conditions (-450 "C and -150 a b ) , indicating the cooperative role of solvent and catalyst in terms of dissolution and depolymerization of coal macin Aduance ACS Abstracts, December 1,1993. (1)Derbyshire, F.;Davis, A.; Schobert, H.; Stansberry, P. Prepr. ACS Diu. Fuel Chem. 1990, 35 (l), 51. (2) Strobel, B. 0.;Friedrich, F. Proc. Int. Conf. Coal Sci. 1989, 735738. e Abstract published
0887-0624/94/2508-0025$04.50/0
romolecules, suppression of retrogressive reactions, and regeneration of the donor solvent.3-5 The present authors revealed that the initial step of liquefaction under lower hydrogen pressure involves dissolution of coal by the donor as a primary route independent of the presence of catalysL6 It should be noted that slow heating in an autoclave is never favorable for the donor to perform effectively in hydrogen-transfer depolymerization.' Two-stage liquefaction processes involving stepwise application of donors and catalysts such as EDS (Exxon donor solvent) and SCT-TSL (short contact time twostage liquefaction) processes in the United States and the LSE (liquid solvent extraction) process in the UK have been investigated as a means to directly produce distillable light oil using Co-Mo or Ni-Mo catalysts in the second stage, although no catalyst was used in the first stage.8 Recently, closed coupled and/or integrated two-stage liquefactions (CC-ITSL) have been investigated to elucidate the effects of thermal/catalytic and catalytic/ (3) Okuma, 0.; Yanai, S.; Tachibana, S.; Hirano, T.; Ida, T. Proc. Znt. Conf.Coal Sci. 1989, 701-704. (4) Satoh, T.; Seventh AIST-NEDO/DOE-PETC Joint Technical Meeting, Coal Liquefaction Proceedings; 1990, p 27. ( 5 ) Gollakota, S. V.; Vimalchand, P.; Davies, 0. L.; Lee, J. M.; Comer, M. C.; Cantrell, C. E. Eighth AIST-NEDO/DOE-PETCJoint Technical Meeting, Coal Liquefaction Proceedings; 1991; p 23. (6) Mochida, I.; Iwamoto, K.; Tahara, T.; Korai, Y.; Fujitau, H.; Takeshita, K. Fuel 1982,61, 603-609. (7) Sakata, R.;Sakanishi, K.; Mochida, I. Znt. Conf. Cod Sci. 1991, 707-710. (8) Whitehurst, D. D.; Mitchell, T. 0.; Farcasiu, M.CoalLiquefaction; Academic Press: New York, 1980.
0 1994 American Chemical Society
Mochida et al.
26 Energy & Fuels, Vol. 8, No. 1, 1994
thermal staging on solids buildup.9 Two-step liquefaction is worthwhile to be examined within the primary liquefaction stage because catalysts and donors do not always perform most efficiently under the same conditions. Hence, optimal application is achieved in consecutivesteps, where the best conditions can be selected for each step separately. In such a two-step primary liquefaction process, expensive catalysts such as Co-Mo and Ni-Mo are not necessarily employed. These catalysts are more appropriately used when the coal has been depolymerized to soluble products and catalyst poisons are not present in the final upgrading stages. The stages of coal conversion to final products are referred to as the primary stage, in which coal is converted to primary liquid (soluble) products in two steps, and a secondary or final stage, in which the primary products are upgraded to the final distillate products. The present authors have been studying a complete conversion of coal (no organic residue) with the least amount of hydrogen donor in the reactor by a multistage scheme which includes the coal pretreatment, coal dissolution, and catalytic upgrading.'OJ1 The catalyst of the primary liquefaction stage is a key to be developed for such a scheme. The authors assumed that the recovery and recycling of the catalyst from the residue constitute an essential approach to reach the objective described above. Combination of donor and catalyst gives a chance to convert whole coal into distillable oil, leaving no organic residue. The complete deashing from the coal allows the recovery of the catalyst. The removal of metal cations is expected to reduce the catalyst deactivation. The removal of metal cations was also found to activate even some of the unreactive macerals such as semi-fusinite in Australian brown and subbituminous coals, which can be at least soluble during the liquefaction by donor.12-14 Such an activation of semi-fusinite by the removal of cations appears to be characteristic to the Gondowanan coals, although the treatment certainly enhances the fusibility and solubility of some c 0 a l s . ~ ~The ~ 8 catalyst recovery and repeated use are possible by leaving no residues other than the catalyst after the liquefaction and by separating the catalyst. The deashing of coal leaving no inorganic residue can be an approach for catalyst recovery. The recovery of catalyst from the residue is designable by physical separations. Dow reported gravity se~arati0n.l~ The magnetic separation is another approach for feasible separation. The catalysts or catalyst supports of ferromagnetism can be designed. In the present study, recovery and recycle of the catalyst for the primary liquefaction stage were studied: The basic (9)Sullivan, R. F. Prepr. Diu. Fuel Chem. ACS 1986,31 (4),280. (10)Mochida. I.: Sakanishi. K.; Korai, Y.; Fujitau, H. Fuel Process. Technot. 1986,i4,'113-124. (11)Sakanishi, K.; Honda, K.; Sakata, R.; Mochida, 1.5th Australian Coal Sci. 1992,97-102. (12)Mochida, I.; Kishino, M.; Korai, Y.; Sakanishi, K. J . Fuel SOC. Jpn. 1986,65,82&834. (13)Mochida, I.; Kishino, M.; Sakanishi, K.; Korai, Y.; Takahashi, R. Energy Fuels 1987,1,343-348. (14)Sakanishi. K.: Towata, A.: Mochida. I . Energy __ Fuels 1988.2.802807. (15) Mochida, I.; Moriguchi,Y.; Shimohara, T.; Korai, Y.; Fujitau, H.; Takeshita, K. Fuel 1983,62,471-473. (16)Mochida, I.; Shimohara, T.; Korai, Y.; Fujitau, H. Fuel 1984,63, 847-851. (17) Mochida, I.; Yufu, A.; Sakanishi, K.; Korai, Y.; Shimohara, T. J. Fuel SOC. Jpn. 1986,65,1020-1026. (18)Larsen, J. W.; Pan, C.-S.; Shawver, S.Energy Fuels 1989,3,557561. (19)Whitehurst, D.D.(Ed.) Coal Liquefaction Fundamentals. ACS Symp. Ser. 1980,139.
idea was to recover the catalyst after the liquefaction from the inorganic residues which come from the feed coal. According to the natures of inorganic residue, two approaches were examined in the present study: (1)washing out the inorganic residues such as carbonates which are principally found in an Australian brown coal (when no inorganic residue is left after the liquefaction, there is no necessity of washing) and (2) recovery of the ferromagnetic catalyst from the diamagnetic residue by applying the magnetic gradient. The catalyst deactivation by carbons and minerals can be minimized by the multistage scheme. The sequences of pretreatment and hydrogen-transferring liquefaction followed by the catalytic steps are responsible. The catalyst and organic residue can be recycled to the primary liquefaction stage when the organic residue still carries significant amount of reactive portions in a similar manner to the bottom recycle.
Experimental Section Materials. Morwell coal (Australian brown coal: C 66.7, H 4.9, N 0.6, ash 2.3 w t %) was used for the liquefaction reaction. The liquefaction (hydrogen-donating) solvent was a hydrogenated fluoranthene prepared by catalytic hydrogenation of commercial fluoranthene (FL)using a commercial Ni-Mo catalyst in an autoclave at 250 O C , under initial hydrogen pressure of 13.5 MPa. The major component of the solvent was 1,2,3,10btetrahydrofluoranthene (IHFL),which was identified by lH and 13C NMR, quantified by GC, and purified by recrystallization with n-hexane, removing perhydrofluoranthenes (PHFL).20z21 Pyrene (Py) of guaranteed grade was used without further purification as a nondonor solvent in some experiments to mix with 4HFL. Catalysts. Catalysts examined in the present study were a synthetic pyrite (
