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Energy & Fuels 1999, 13, 710-718
Polyolefin Degradation in a Continuous Coal Liquefaction Reactor Kurt S. Rothenberger* and Anthony V. Cugini U.S. Department of Energy, Federal Energy Technology Center, P.O. Box 10940, Pittsburgh, Pennsylvania 15236
Robert L. Thompson Parsons Infrastructure & Technology Group Inc., P.O. Box 618, Library, Pennsylvania 15129
Ronald J. Pugmire and Mark S. Solum Department of Chemical and Fuels Engineering, 210 Park Building, University of Utah, Salt Lake City, Utah 84112 Received October 20, 1998
A novel solvent extraction method to isolate and recover polyolefin materials from coal-plastics coprocessing product streams is reported. The method was applied to samples obtained from a bench-scale continuous unit, coprocessing coal with polyethylene (PE), polypropylene (PP), and polystyrene (PS) feed. Recovered PE and PP have been characterized by infrared (IR) and nuclear magnetic resonance (NMR) spectroscopies, and gel permeation chromatography (GPC); PS is completely converted to distillable product. The results indicate that PP undergoes fairly rapid and essentially quantitative reaction and its conversion is complete before reaching the downstream portion of the process. On the other hand, PE undergoes some degradation in the coal liquefaction reactor, with an average reduction in molecular weight distribution for the “unconverted” material by a factor of 10 to 30. GPC can definitively distinguish between fresh (feed) and recycled PE in the process stream and has established that most of the PE degradation occurs in the first-stage liquefaction reactor. This partially converted, but undistillable material then passes into the atmospheric still bottoms stream. The two solid separation methods examined had very different effects on the incompletely reacted PE. Vacuum distillation sequesters the PE in the unconvertable (ashy) fraction, whereas pressure filtration allows most of it to pass through into the recycle stream. A qualitative mechanism for PE breakdown is proposed in which rapid scission occurs at the branching points of the paraffin backbone, followed by eventual breakdown to distillable products
Introduction Although polyethylene (PE) is generally thought of as a 20th century material, the first polyethylene-like product was prepared in 1898, from the decomposition of diazomethane.1 High-pressure synthesis of PE from ethylene gas as we know it was accomplished in 1933, but the technology and market for PE did not blossom until the advent of Ziegler catalysts in the 1950s.2,3 Today, PE is one of the dominant polymeric hydrocarbon materials, with U.S. production for combined low- and high- density PE reaching some 26 billion pounds, or about 40% of total thermoplastic resin production, in 1996.4 The usefulness and versatility of plastics as packaging materials and in consumer products makes * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) von Pechmann, H. Ber. 1898, 31, 2643. (2) Encyclopedia of Polymer Science and Technology; WileyInterscience: New York, 1967; Vol. 6, pp 276-277. (3) Billmeyer, F. W., Jr. Textbook of Polymer Science; WileyInterscience: New York, 1962; p 363.
them a growing component of municipal waste streams. High-density PE has proliferated in such diverse materials as food containers and patio furniture. Recent statistics indicate that low- and high-density PE together make up about half of all municipal plastic waste.5 Studies of the degradation of PE are nearly as old as those of its synthesis. Upon heating in the absence of oxygen, PE has been observed to degrade at temperatures as low as 290 °C,6 although modern, thermallyresistant PE and the use of additives has increased this threshold. Recently, processes for pyrolysis and cracking of PE for conversion to transportation fuels were (4) Stork, W. J.; Layman, P. L.; Reisch, M.; Thayer, A. M.; Kirschner, E. M.; Peaff, G.; Tremblay, J. F. Chem. Eng. News 1997, 75 (25) (June 23, 1997), 40-45. (5) Smith, R. A. “Overview of Feedstock Recycling of Commingled Waste Materials,” Ninth Annual Technical Meeting, Consortium for Fossil Fuel Liquefaction Science, Pipestem, WV, August 15-18, 1995. (6) Madorsky, S. L. Thermal Degradation of Organic Polymers, Polymer Reviews, Vol. 7; Mark, H. F., Immergut, E. H., Eds.; WileyInterscience: New York, 1964; pp 93-129.
10.1021/ef980230n CCC: $18.00 © 1999 American Chemical Society Published on Web 04/22/1999
Polyolefin Degradation
published employing temperatures of 400 °C and above.7,8 Unlike polymers such as polystyrene (PS) which “unzip” to form large amounts of monomer under pyrolysis conditions, PE generally undergoes random cracking because all of the C-C bonds in PE (except for those at the chain ends and branch points) are of equal strength and have the same probability of breaking as energy is added to the system. In the presence of catalyst, cracking may be followed by rearrangement to a complex mixture of alkanes, olefins, and aromatics.6,7 Polypropylene (PP) emerged as an important polymer later in the 1950s, as described in an excellent historical and technical review by Pino and Mulhaupt.9 Previously, propylene was polymerized with metal oxides, but the little practical interest in the reaction was due to the small quantities of solid material obtained, probably due to a large number of diastereomers in the mixture produced. Not until 1955 did Natta first prove the existence and report the properties of isotactic polypropylene, and the potential value of the material could begin to be realized. Over the last 10 years, the market for PP has grown even faster than that for PE, reaching nearly 12 billion pounds of thermoplastic resin in 1996.4 Although thermal degradation of PP is more facile than that for PE,6 its growing usage and subsequent presence in the municipal waste stream make its inclusion necessary in any study of issues involving plastic waste. The coprocessing of coal with waste materials such as plastic has shown promise as a means to lower the cost of producing liquid fuels from coal and simultaneously recover the inherent value of the wastes.10 However, recent studies have demonstrated that highdensity PE can be quite difficult to convert in a traditional coal liquefaction environment employing hydrogenation catalysts and donor solvents.10,11 Some of the best PE conversion results have been obtained using either lower-density PE, or by employing cracking catalysts which would tend to foul in the presence of coal-derived species.12,13 The coal liquefaction environment that has evolved under the U.S. Department of Energy’s Catalytic Multi-Stage Liquefaction (CMSL) program14 is quite different than that encountered during thermal or catalytic pyrolysis. Understanding the degradation behavior of PE and PP in a liquefaction environment and in the presence of coal is important to the development of a successful scheme for coprocessing coal with plastics. This paper discusses a novel analytical method developed at Federal Energy Technology Center (FETC) to recover incompletely reacted polyolefins from coalwaste coprocessing product streams. The technique was applied to sample streams obtained from six different (7) Ng, S. H. Energy Fuels 1995, 9, 735-742. (8) Al-Amrousi, F. A. Fuel 1997, 76, 1451-1457. (9) Pino, P.; Mulhaupt, R. Angew. Chem., Int. Ed. Engl. 1980, 19, 857-875. (10) Coal and Waste; Huffman, G. P., Anderson, L. L., Eds.; Fuel Processing Technology Vol. 49; Elsevier: Amsterdam, 1996. (11) Rothenberger, K. S.; Cugini, A. C.; Thompson, R. L.; Ciocco, M. V. Energy Fuels 1997, 11, 849-855. (12) Taghiei, M. M.; Feng, Z.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1994, 8, 1228-1232. (13) Ding, W.; Liang, J.; Anderson, L. L. Energy Fuels 1997, 11, 1219-1224. (14) Catalytic Multi-Stage Liquefaction (CMSL); U.S. Department of Energy, National Technical Information Service, Springfield, VA, 1996; DE-AC22-93PC92147.
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locations in a continuous bench-scale coal liquefaction reactor operating under the U.S. Department of Energy’s CMSL program, and processing various proportions of coal and model plastic waste. The recovered polyolefins were characterized by gel permeation chromatography (GPC) and solid-state 13C nuclear magnetic resonance (NMR) spectroscopy; these results revealed important information about the reactivity of PE and PP in a continuous coal liquefaction process. The results indicate that PP undergoes fairly rapid and essentially quantitative reaction and is not a factor in the downstream portion of the process. On the other hand, PE undergoes some degradation in the coal liquefaction reactor, with an average reduction in molecular weight distribution for the “unconverted” material by a factor of 10 to 30. A qualitative mechanism for PE breakdown is proposed in which rapid scission occurs at the branching points of the paraffin backbone, followed by eventual further breakdown to distillable products. Experimental Section Materials. Liquefaction experiments were conducted using subbituminous Black Thunder mine coal (HRI6213), ground to -60 mesh. High-density PE (Tm ) 135 °C, d ) 0.96 g/mL) was supplied by Solvay Polymers. According to the manufacturer, this PE (catalog no. T503600) contained a small amount of hexene, resulting in approximately six butyl branches per 1000 carbons of polymethylene backbone. PS (Tm ) 95 °C, d ) 1.0 g/mL) was supplied by BASF. PP (Tm ) 176 °C, d ) 0.94 g/mL) was supplied by AMCO Plastics. All plastics were supplied as 3.2 mm (0.125 in.) extruded pellets. A mildly hydrogenated petroleum-derived oil containing small amounts of coal-derived liquid (L-814) was used as a start-up solvent in the continuous reactor, but it was operated in a recycle mode thereafter. Plastics were not introduced into the system until the 30th day of the run, at which time the system was presumed to be operating on a completely coal-derived recycle solvent. MOLYVAN A, an inexpensive commercial lubricant additive, and Fe (prepared as FeOOH/SO4) were used as catalysts during the run. Tetrahydrofuran (THF) and dichloromethane solvents used in workup and extraction procedures were obtained in bulk grade and used without further purification. Decane fraction (bp 171-177 °C), used in the PE recovery procedure, was obtained from Fluka Chemie AG and used without further purification. Reactions. Continuous unit tests were performed on a bench-scale, close-coupled, two-stage, catalytic reactor system at Hydrocarbon Technologies, Inc. (HTI) as part of the U.S. Department of Energy’s CMSL program.14 Samples were obtained from run CMSL-9, which was configured with a pretreater, two liquefaction reactors, and an on-line hydrotreater, processing approximately 35 kg/day of feed material under a back pressure of 2500 psig. The run was made in an all-dispersed catalyst mode using 300 ppm Mo and 10 000 ppm Fe (on a solid feed basis) introduced into the pretreater as MOLYVAN A and FeOOH/SO4, respectively. Additional process details may be found in the final technical report.14 Samples were analyzed during four different coprocessing run conditions, identified by the feed type: a baseline feed condition (actually period 29 of run CMSL9), consisting of coal only (without plastics), condition
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Figure 1. Schematic diagram of the bench-unit configuration: (a), feed slurry vessel; (b), pretreater; (c), first-stage reactor; (d), second-stage reactor; (e), hot separator; (f), on-line hydrotreater; (g), cold separator; (h), atmospheric still; (j) solid separation device (either vacuum still or pressure filter). Sampling points, indicated in bold capital letters, were available for the feed slurry (FS), pretreater (PTR), first-stage reactor (RCT), distillate product (DIST), atmospheric still bottoms (ASB), vacuum still bottoms (VSB) or pressure filter solid (PFS), and vacuum still overhead (VSOH) or pressure filter liquid (PFL) process streams.
1 (period 34) consisting of coal and mixed plastics in a 2:1 ratio (67% coal, 13% PE, 11% PP, 9% PS), condition 2 (period 38) consisting of coal and PE in a 2:1 ratio (67% coal, 33% PE), and condition 3 (period 41) consisting of coal and mixed plastics in a 1:1 ratio (50% coal, 20% PE, 16.5% PP, 13.5% PS). A simplified schematic diagram of the continuous unit configuration during this run is shown in Figure 1. Samples were obtained from the following points, identified in Figure 1: (FS) feed slurry, (PTR) pretreater, (RCT) first-stage liquefaction reactor, (ASB) atmospheric still bottoms, (VSOH) vacuum still overhead (condition 1 only), (PFL) pressure filter liquid (conditions 2, 3 only), (VSB) vacuum still bottoms (condition 1 only), and (PFS) pressure filter solids (conditions 2, 3 only). Extraction of Polyolefinic Material from BenchScale Continuous-Unit Process Streams. To more thoroughly investigate the behavior of PE and PP in a coal liquefaction system, a general method was devised to recover polyolefinic material from coal liquefaction process streams. The method is diagrammed in Figure 2. The first step involved a cold THF wash to remove as much soluble coal-derived material as possible without affecting the incompletely reacted PE or PP. In fact, this step alone was sufficient for isolating PE from nonashy recycle streams, i.e., those in which the insoluble coal and mineral matter had already been removed.15 The THF insolubles were then subjected to a hot decane extraction; the decane solubilized the incompletely reacted PE or PP, leaving the coal-derived solids behind. The incompletely reacted PE or PP was (15) Pradhan, V.; Robbins, G. Personal communication.
Figure 2. Schematic diagram of the extraction recovery process.
recovered from the decane as a powder following cooling and filtration. The recovered powdery solid was washed with dichloromethane to remove entrained solvent and residual coal-derived material, leading to the isolation of the incompletely reacted PE or PP as a beige powder. The method was applicable to a wide range of process streams including tars, solids, and multiphase mixtures. Samples obtained from the previously described streams of the continuous coal liquefaction reactor were received in forms ranging from solids to viscous tars. If the sample was a solid material, it was crushed in a mortar and pestle prior to the extraction procedure. If the sample was a tar, it was heated in an oven at low temperature (
