Energy & Fuels 1994,8, 1228-1232
1228
Coliquefaction of Waste Plastics with Coal M. Mehdi Taghiei, Zhen Feng, Frank E. Huggins, and Gerald P. Huffman* Consortium for Fossil Fuel Liquefaction Science, 341 Bowman Hall, University of Kentucky, Lexington, Kentucky 40506-0059 Received March 28, 1994. Revised Manuscript Received August 10, 1994@
Polyethylene (PE), poly(ethy1ene terephthalate) (PET), polypropylene (PPE), and actual plastic wastes from such items as milk jugs, soft drink bottles, plastic wraps, plastic flatware, etc., have been successfully converted to oil in direct liquefaction experiments with coal. Comparative experiments were performed with and without the presence of coal under typical direct liquefaction conditions (420-450 "C, 60 min reaction time, 800 psig of Hz cold). Two types of catalysts were used: highly dispersed iron-based catalysts, and an HZSM-5 zeolite catalyst. Using PE, PPE, PET, and a mixed waste plastic with the zeolite catalyst, oil yields of 80-98% and total conversions of 90-100% were obtained at liquefaction temperatures of 420-430 "C. A nanoscale ferrihydrite catalyst in a sulfided state was less active but also gave similar results at somewhat higher temperatures. Coliquefaction experiments were performed on coal-plastic mixtures (usually 50: 50 mixtures) using a bituminous coal, a subbituminous coal, and a lignite. The HZSM-5 zeolite catalyst and nanoscale iron catalysts were used separately and together. The oil yields for these coliquefaction experiments were as high as 60-80%, while the total conversions reached levels of over 90%. Oil yields for coal-plastic mixtures were higher, typically by -lo%, than the average of the oil yields for the coal and plastic alone, implying synergistic effects.
Introduction Since their introduction, plastic products have proven to be efficient, inexpensive, and popular with consumers. Today, about 30 million tons of plastics are produced in the US. every year, one-quarter of which is used in the single-use packaging market.l Currently, most plastic products are discarded after use and end up in sanitary landfills. Waste plastics occupy about 11%by weight and 21% by volume of U S . landfills. In the last decade, about 40% of the landfills in the U.S. were closed and half of the remaining landfills are expected to be filled by the end of the century. Despite this anticipated shortage, there is great resistance in most communities to developing new landfill sites. For example since 1981, the city of New York has tried to develop five new landfill sites, without success.2 Hence, it is clear that landfills can no longer continue to be the major means for disposal of plastic wastes and such disposal will become an increasingly serious environmental concern. Recycling of materials can play an important role in protecting our environment and conserving valuable natural resources and plastics recycling would appear to be a possible alternative solution to disposal in landfills. However, the recycling of plastics that takes place currently is not due to industry initiatives, but a result of government edicts. In the United States, laws are in effect in states such as California, Oregon, and Wisconsin requiring plastic bottles to be made from at least 25 wt % of recycled plastic. Similar laws in other states and federal laws are likely to follow. In Germany, the law requires that 80% of plastic wastes must be recycled by 1995. France recently passed a similar law. * Author to whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, September 15,1994.
(1)Modern Plastic, 1990,120.
(2) Alexander, J. H. I n Defense of Garbage; Paeger Publishers: CT,
1993.
0887-0624/94/2508-1228$04.50/0
Conventional plastic recycling is facing a number of problems. There are nearly a hundred classes of plastic polymer with a thousand specifications. Therefore, an advanced technology is required for sorting and separating postconsumer waste plastic for high-quality recycling. Moreover, the potential exists for impurities to diffuse into products packaged in recycled containers. Conventional plastic melting temperatures are not high enough to eliminate all possible contaminants in waste plastics. Therefore, the U.S. Food and Drug Administration (FDA) has banned the use of recycled plastics for products that come in direct contact with food. Consequently, plastic molding companies have to make recycled plastic bottles from several layers of plastics, of which the inner layer must be made from virgin polymer. Aside from health concerns, contaminants in recycled plastics may damage the processing equipment. Hence, the equipment conversion or replacement costs in order to use reclaimed plastics can be much higher. As a result of all these problems, the average price of recycled high-density polyethylene is 10% higher than Consequently, that of virgin high-density p~lyethylene.~ out of 30 million tons of total plastic generated each year in the U.S., only about 4% is currently re~ycled.~ Another option for plastic waste disposal is incineration. The incineration of waste materials in which plastic wastes along with other wastes are combusted in a furnance currently accounts for about 10% of the total municipal waste disposal in the U.S. Although such incineration facilities are equipped with air pollution control devices, such as electrostatic precipitators, fabric filters for particulate control, and dry or wet scrubbers for acid gas removal, there is still substantial public concern over environmental issues regarding (3) Graff, Gordon. Modern Plastics, 1992,45. (4) Characterization ofMunicipaZ Waste in the United States. U.S.
Environmental Protection Agency, Washington, DC, July 1992.
0 1994 American Chemical Society
Coliquefaction of Waste Plastics with Coal
incineration. Such concerns not only add t o the high cost of pollution control but also result in significant uncertainty regarding the construction and future operation of these facilities. Hodek5 studied conventional pyrolysis as an alternative for plastic waste recycling. However, he found that this method usually results in unsaturated and unstable oils of low yield and low value that can be used only for combustion. Van Heek et a1.6 studied the pyrolysis of shredded plastic waste over a range of temperature from 300 t o 800 "C. The amount of oil produced at 300 "C is less than 2%, increases to 20% at 500 "C, and then decreases to below 10% at 800 "C. Considering current conditions and expected future trends, none of the above options for plastics disposal is entirely satisfactory. Several recent paper^,^-^ however, indicate that direct liquefaction should be considered not only as a solution for waste plastics disposal but also for generating an environmentally acceptable transportation fuel or feedstock for the production of virgin plastics. The current rate at which waste plastic materials are produced in the United States each year constitutes a potential hydrocarbon resource from which over 80 million barrels of oil per year could be produced. Moreover, coliquefaction of plastic wastes with aromaticrich materials such as coal could yield synergistic effects and increase the production of oil from this source. In this study, we explore the direct liquefaction of medium-density and high-density polyethylene (MDPE and HDPE), polyethylene terephthalate (PET), and polypropylene (PPE), as well as actual postconsumer waste plastics, both alone and mixed with several coals. The effects of two types of potential catalysts (HZSM-5 zeolite and ultrafine iron-based catalysts1°-13) are examined.
Experimental Section Medium-density polyethylene (MDPE), high-density polyethylene (HDPE), polflethylene terephthalate) (PET), polypropylene (PPE), and actual plastic wastes, such as milk jugs, and soft drink bottles, and a mixed waste plastic (MWP) prepared in our laboratory from a variety of items (milk jugs, soft drink bottles, yogurt containers, motor oil bottles, disposable plastic flatware, plastic sacks and wraps, etc.) were liquified in this study. Coliquefaction experiments were performed on mixtures of MDPE and the MWP with Blind Canyon bituminous coal, iron ion-exchanged Beulah lignite,1° and iron ion-exchanged Black Thunder subbituminous coa1.l' Proximate and ultimate analyses for these coals and the mixed waste plastic (MWP)prepared in our laboratory are shown in Table 1. The experiments used several types of catalysts: ion(5)Hodek, W. Proc.,Int. Conf. Coal Sci.,Newcastle-Upon-Tyne, UK, 1991,782-785. (6)Van Heek, K. H.; Strobel,B. 0.;Wand, W. Presented at the Fuel Conference, "Coal Utilization and the Environment", 17-20 May 1993, Orlando, FL. (7) Strobel, B. 0.;Dohms, K.-D. Proc. Int. Conf. Coal Sci., II 1993, 536-539. (8)Taghiei, M.M.; Huggins, F. E.; Huffman, G. P. Prepr. Pap.-Am. Chem. SOC.,Diu. Fuel Chem. 1993,38(4), 810-815. (9)Anderson, L. L.; Tuntawiroon,W. Prepr. Pap.-Am. Chem. SOC., Diu.Fuel Chem. 1993,38(4),816-822. (10)Taghiei, M. Mehdi; Huggins, F. E.; Ganguly, B.; Huffman, G. P.Energy Fuels 1993,7,399. (11)Taghiei, M. Mehdi; Huggins, F. E.; Mahajan, V.; Huffman, G. P.Energy Fuels 1994,1, 31. (12)Zhao, J.;Feng, Z.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1994,1 , 38. (13)Zhao, J.;Feng, 2.; Huggins, F. E.; Huffman, G. P. Energy Fuels, in press.
Energy & Fuels, Vol. 8, No. 6,1994 1229 Table 1. Proximate and Ultimate Analyses of Coal Samples and Waste Plastics Used in This Research Black Blind Beulah Thunder Canyon mixed waste lignite coal coal plastic proximatea % ash 9.56 6.34 6.57 0.45 % volatile 46.75 98.80 56.08 43.62 % fixed carbon 46.68 0.74 34.36 49.83 ultimateb % carbon 73.14 73.49 81.61 84.65 4.47 % hydrogen 6.21 13.71 3.09 1.01 % nitrogen 1.38 0.65 1.29 0.60 % total sulfur 0.47 0.01 0.77 20.61 % oxygen 19.33 10.33 0.98 a Dry basis. Dry mineral matter free basis. exchanged iron,lOJ1ultrafine ferrihydrites,12J3and an HZSM-5 zeolite catalyst. The ion-exchange experiments have been described in detail elsewhere.l0Jl Briefly, a freshly made 0.05 M aqueous solution of ferric acetate was controlled t o a pH value of about 2.8 using sulfuric acid at a temperature of 60 "C in a 10 L fermenter. A slurry made from a 10:1 ratio by weight of dry coal and ferric acetate solution was stirred under a nitrogen atmosphere for various contact times. At the completion of the procedure, the ion-exchanged coal samples were repeatedly washed with distilled water until the pH value of the filtrate for two consecutive washes was constant. Most of the liquefaction experiments were conducted in 50 mL microautoclave reactors (tubing bombs). The reactors are first charged with 5 g of feedstock (plastic or coal plus plastic). Tetralin was added with feedstock in most experiments in a 3:2 tetra1in:feedstock ratio. Several experiments were run using a waste motor oil as a solvent (waste oi1:plastic ratio = 3:2). Unless otherwise noted, the zeolite and ferrihydrite catalysts were added at a concentration level of 1wt % of the feedstock. In order to sulfidize the iron when iron catalysts were present, dimethyl disulfide (DMDS) was added with a sulfur to iron ratio of 2. The reactors are then pressurized to 800 psig of hydrogen (cold) and heated t o the desired temperature in a fluidized sand bath while being agitated at 400 cycles/min. At the end of the reaction period, the microreactor is quenched in a room temperature sand bath. The liquefaction products are determined by Soxhlet extraction, and the gaseous products are collected and analyzed by gas chromatography. The liquefaction products are separated into oil (THF and pentane soluble), asphaltene and preasphaltene (THF soluble, but pentane insoluble), and insoluble residues or IOM (THF insoluble). Total conversion is defined as { 100 - % (THF insolubles)}, while oil yield is defined at (100 - % (pentane insolubles) - % (THF insolubles) - % (gas)}. Although the gas yield was not determined for some experiments, it was generally quite low ( < 1 to 2%). PET was the only feedstock exhibiting a significant gas yield (13-14%). To assess the reproducibility of the experiments, two independent microautoclave apparatuses (different tubing bombs, shakers, sand baths, temperature controllers, etc.) were used by two different operators. Total conversions and oil yields determined in experiments run by a single operator on a single system were normally repeatable to within &l%, while those measured by different operators on different systems were repeatable to within 4~5%.A conservative estimate of the accuracy of the data presented is therefore &5%. Soxhlet extraction results for various plastic resins and the MWP prior to reaction are summarized in Table 2. I t is seen that the MDPE exhibited significant solubility in THF prior to reaction, which increased with extraction time. The pentane solubility of MDPE is small but not negligible. The extraction percentages for all of the other resins and the MWP are seen to be quite low (
