Energy & Fuels 1996, 10, 733-742
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Hydrocracking Reactivities of Primary Coal Extracts Prepared in a Flowing-Solvent Reactor Sheng-Fu Zhang, Bin Xu, Alan A. Herod, and Rafael Kandiyoti* Department of Chemical Engineering and Chemical Technology, Imperial College, University of London, London SW7 2BY, U.K. Received September 26, 1995. Revised Manuscript Received December 13, 1995X
Structural changes taking place during the hydrocracking of coal liquefaction extracts prepared in a flowing-solvent reactor (FSR) have been studied. Preparation of coal extracts in this type of reactor allows determination of hydrocracking reactivities in isolation from secondary effects associated with long product residence times during the coal dissolution step. Parallel hydrocracking experiments have been carried out with a pilot-plant (PP) extract which had a different process history. A TGA (thermogravimetric analysis)-based method for determining boiling point distributions of heavy coal-derived liquids has been described. The method has proved useful for mixtures not amenable to gas chromatography based simulated distillation. Products have also been characterized by size exclusion chromatography and UV-fluorescence spectroscopy using 1-methyl-2-pyrrolidinone as the solvent. Comparison with results obtained from these two techniques using tetrahydrofuran (THF) as solvent has indicated partial loss of sample in THF, due to poorer solubility. Compared to the pilot-plant extract, a higher proportion of the +450 °C bp material present in the FSR extract was found to break down in the hydrocracking stage. Parts of the +450 °C bp material in the extracts appear to convert to chemically more stable, large molecular-mass structures with increasing intensity and length of processing. At hydrocracking temperatures between 440 and 460 °C, the most significant structural changes in the FSR extracts have been observed during the first 30 min. Analogous structural differences between the PP extract and its hydrocracking products during the first 30 min were somewhat smaller. Progressively smaller changes in boiling point distributions, SEC chromatograms, and UV-F spectra have been observed between 30 and 150 min. The data do not allow distinguishing between diminishing sample reactivity and a drop in catalyst activity, but at least part of the slow-down in conversion appears due to progressive reduction in the reactivity of the extracts.
Introduction At temperatures above 400 °C, the dissolution of most coals is a rapid process, usually completed within 6-7 min.1 However, in batch or continuous stirred reactors, considerably longer residence times of primary extracts in the reaction zone are common, allowing products to undergo secondary reactions. Depending on reaction conditions in the digester, these reactions might include retrogressive repolymerizations and/or reactions leading to molecular mass reduction,2 viz. cracking, dehydroxylation, and dealkylation. Coal extracts with relatively long residence times in the digester are therefore likely to represent a mixture of products from cascades of secondary reactions. The use of catalysts in coal dissolution also tends to mask distinctions between primary extracts and products of secondary reactions, usually through catalyzed hydrocracking of extracts already released from the coal. When a separate hydrocracking stage is installed as part of a proposed process scheme, it is usually sufficient to evaluate the hydrocracking reactivity of the stream entering the hydrocracker, in order to evaluate perforX Abstract published in Advance ACS Abstracts, February 1, 1996. (1) Xu, B.; Madrali, E. S.; Wu, F.; Li, C.-Z.; Herod, A. A.; Kandiyoti, R. Energy Fuels 1994, 8, 1360. (2) Gibbins, J. R.; Kimber, G.; Gaines, A. F.; Kandiyoti, R. Fuel 1991, 70, 380.
0887-0624/96/2510-0733$12.00/0
mance within the parameters of the process design. This type of experiment, however, contributes relatively little to fundamental understanding of the nature and sequence of chemical reactions required to produce saleable products from coal extracts. Clearly, not only the yields and structures but also the chemical reactivities of the extracts are altered by relatively poorly controlled extraparticle secondary reactions taking place in the presence and during the dissolution of the coal. Efforts to improve process design require more detailed examination of the effects of hydrocracking reaction conditions on structural changes of coal extracts, in isolation from secondary reactions. Furthermore, in attempting to compare hydrocracking reactivities of coal extracts on a common basis, it is necessary to take account of reaction conditions prevailing during the initial coal dissolution step. This paper describes hydrocracking experiments carried out using coal extracts prepared in a reactor, where extraparticle reactions of materials released from coal particles have been minimized. The reactor consists of a fixed bed of coal, swept by a continuous stream of solvent, with products carried out of the heated zone within 6-10 s and rapidly quenched. This method of preparation allows the determination of extract hydrocracking reactivities in isolation from secondary effects associated with the coal dissolution step. Hydrocracking experiments have been carried out in a microbomb © 1996 American Chemical Society
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Table 1. Coal Extracts Used in the Hydrocracking Experiments extract PP extract FSR extract
description coal extract solution from Point of Ayr Pilot Plant 5 K s-1, 450 °C, 400 s, F-S extract, PI
+450 °C (wt %)a 63.9 72.7
a
Proportion of bp > 450 °C material in coal extract (PI) determined by the thermal gravimetric analysis (TGA) based method.
reactor with close control of final pressure at peak temperature. Parallel hydrocracking experiments have been carried out on an extract from the Point of Ayr pilot-plant facility3 where mean residence times of extracts in the digester are of the order of 1 h (Table 1). A comparison of conversions and changes observed in product structures as a function of reaction conditions will be presented. Analytical steps have been formulated to deal with problems concerning small sample sizes used in the hydrocracking experiments (about 200 mg) and those associated with distinguishing between coal-derived products and solvent (tetralin)-derived materials. Product characterization by size exclusion chromatography (SEC) and UV-fluorescence spectroscopy (UV-F) has indicated that the use of tetrahydrofuran (THF) as solvent does not allow detection of the full range of products, compared to 1-methyl-2-pyrrolidinone (NMP).4-7 Characterization by SEC and UV-F has been undertaken using NMP. A method for determining boiling point distributions based on thermogravimetric analysis will also be described. Experimental Section Liquefaction of Coal. Point of Ayr coal (UK; C, 84.5; H, 5.4; N, 1.8; S, 1.5; O, 6.1% w/w daf; ash, 9.6% db) was ground, sieved, and stored under nitrogen; the 100-150 µm fraction was used in the experiments. The flowing-solvent reactor (FSR) assembly, its control system, and the experimental procedure have described elsewhere.8-10 Briefly, the reactor consists of a two-stage direct electrically heated tubular vessel. The fixed bed of sample (200 mg of coal mixed with about 3 g of sand) is positioned in the upper part of the reactor. A stream of distilled tetralin (0.9 mL s-1) is continuously fed to the reactor at 70 bar. The stream of solvent is preheated in the lower section, to the temperature of the upper (reactor) section, and passed through the fixed bed of sample. In this work, samples were heated at 5 K s-1 to 450 °C and held at peak temperature for 400 s. Extracts released from coal are swept out of the heated zone within 6-10 s and quickly quenched. The solid residue remaining in the reactor was washed with chloroform/methanol solution, and any recovered material was added to the product mixture. (3) Harrison, J. S.; Kimber, G. M.; Gray, M. D. Proc. Int. Conf. Coal Sci., Tokyo 1989, 655. (4) (a) Herod, A. A.; Johnson, B. R.; Bartle, K. D.; Kandiyoti, R. Coal Science; Parajes, J. A., Tasco´n, J. M. D., Eds.; Elsevier, Amsterdam, 1995; Vol. I, p 315. (b) Herod, A. A.; Johnson, B. R.; Bartle, K. D.; Carter, D. M.; Cocksedge, M. J.; Domin, M.; Kandiyoti, R. Rapid Commun. Mass Spectrom. 1995, 9, 1446. (5) Zhang, S.-F.; Xu, B.; Herod, A. A.; Dugwell, D. R.; Kandiyoti, R. Coal Science; Parajes, J. A., Tasco´n, J. M. D., Eds.; Elsevier: Amsterdam, 1995; Vol. II, p 1487. (6) Bartle, K. D.; Herod, A. A.; John, P.; Johnson, B. R.; Johnson, C. A. F.; Kandiyoti, R.; Parker, J. E.; Smith, G. P. Coal Science; Parajes, J. A., Tasco´n, J. M. D., Eds.; Elsevier: Amsterdam, 1995; Vol. II, p 1435. (7) Herod, A. A.; Kandiyoti, R. Coal Science; Parajes, J. A., Tasco´n, J. M. D., Eds.; Elsevier: Amsterdam, 1995; Vol. I, p 949. (8) Gibbins, J. R.; Kandiyoti, R. Rev. Sci. Instrum. 1991, 62(9), 2234. (9) Xu, B.; Dix, M.; Kandiyoti, R. Rev. Sci. Instrum. 1995, 66, 3966. (10) Gibbins, J. R.; Kandiyoti, R. Fuel 1991, 70, 909.
Product mixtures were vacuum distilled (50 °C) to remove light solvent, most of the excess tetralin, and products of the thermal reaction of tetralin.11 Extraction in pentane was carried out to remove mainly tetralin and related products, using 20:1 v/v pentane:concentrated extract solution. Due to suppression of extraparticle reactions of extracts in the flowing-solvent reactor,2 less than 1-2% of coal-derived products in the reaction mixture are normally found to be pentane soluble.12 The dried pentane-insoluble material was recovered as a dark-colored powder. The “Pilot Plant extract” was prepared at the British Coal Point of Ayr Liquefaction Facility.3 In this process, the digestion stage is carried out under the vapor pressure of the recycle solvent (about 30 bar) with no added H2. Catalytic hydrocracking is carried out in ebulliated bed reactors under 200 bar of H2 pressure at 430-450 °C with a 1 h average residence time, in the presence of a NiMo/Al2O3 catalyst. The product is then distilled and the heavy residue, containing up to 20% of saturated polynuclear aromatic hydrocarbons,13,14 is then recycled to the digester as the coal solvent. Hydrocracking Experiments. Extracts were hydrocracked in the microbomb reactor system shown in Figure 1. The reactor was made of a 3/8 in. bored-through Swagelok union tee and connected to the control head via a 1/4 in. o.d. pressure line. The top of the pressure line was water-cooled by a small heat exchanger, serving to condense evaporated solvent and to protect the PDCR 900 remote pressure transducer from exposure to temperatures above 80 °C. A sand fluidized bath was used for heating; reactor temperature was monitored by a Type K thermocouple inserted through the pressure line. A reactor shaker assembly was used for stirring the reaction mixture at 100 cycles/min. The usual assortment of relief and nonreturn valves were installed to ensure safe operation. Reactors with volumes between 3 and 20 mL have been used; in 3 mL reactors, heatup to 450 °C took less than 3 min. Each experiment was carried out with 200 mg of coal extract and 1 g distilled tetralin, in the presence of a fresh batch of 100 mg of commercial presulfided NiMo/Al2O3 catalyst (PBC-90D; Table 2), crushed to less than 250 µm. The catalyst was presulfided by the supplier. Reactions were carried out at 460 °C under 190 bar of H2 pressure for 30, 60, 90, 120, and 150 min. Product Characterization. Figure 2 presents the product preparation scheme in outline. Reactor contents were vacuum filtered and washed with a 4:1 chloroform/methanol solution. The filtrate was transferred to a preweighed sample bottle and purged with nitrogen to evaporate the wash solvent; the weight of the liquid product (in tetralin) was determined. To ensure a homogeneous sample and improve reproducibility of the TGA-based weight loss determination (see below), the liquid product was redissolved in 0.5 mL of chloroform/methanol (4:1 vol/vol) solution before loading on the TGA pan. Boiling Point Distributions by TGA. Due to interference from products of the thermal reactions of tetralin, GC analysis of hydrocracking products did not allow quantitative determination of shifts to lighter boiling materials by simulated distillation methods. Furthermore, volatile polar materials which are not amenable to elution by gas chromatography and larger (>500 u) molecular mass materials with low volatility cannot be detected by this method. Instead, boiling point distributions and proportions of +450 °C bp material in the coal extracts and in the hydrocracking products were determined by TGA, following calibration of the temperature scale against known boiling points of a set of pure compounds. (11) Brodzki, D.; Djega-Mariadassou, G.; Li, C.-Z.; Kandiyoti, R. Fuel 1994, 73, 789. (12) Brodzki, D.; Abou-Akar, A.; Dje`ga-Mariadassou, G.; Li, C.-Z.; Xu, B.; Kandiyoti, R. Fuel 1994, 73, 1331. (13) Wilson, R. Ph.D. Thesis, Heriot-Watt University, Edinburgh, 1993. (14) Wilson, R.; Parker, J. E.; Johnson, C. A. F.; Herod, A. A. Org. Mass Spectrom. 1987, 22, 115.
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Figure 1. Schematic diagram of the microbomb reactor system: (1) microbomb reactor; (2) sand bath heater; (3) shaker; (4) flexible tube; (5) heat exchanger; (6) ball valves; (7) needle valve; (8) thermocouple 1; (9) pressure relief valve; (10) thermocouple 2; (11) pressure transducer; (12, 13, 14) temperature indicators; (15) pressure indicator; (16) motor controller; (17) temperature controller; (18) amplifier; (19) computer; (20) printer; (21) extender; (22) motor; (23) thermocouple 3; (24) heating elements; (25) pressure regulator; (26) flow meters; (27) hydrogen cylinder; (28) gas compressor. Table 2. Chemical Composition and Physical Properties of the Presulfided NiMo/Al2O3 (PBC-90D) Catalyst MoO3 (wt %) NiO (wt %) surface area (m2 g-1) pore volume (cm3 g-1) pore size (Å) size (µm)
8 4 130 0.4 135
