Energy & Fuels 1998, 12, 925-928
925
Classification of Volatile Products from the Temperature-Programmed Pyrolysis of Low- and High-Density Polyethylene Levent Ballice,* Mithat Yu¨ksel, and Mehmet Sagˇlam Faculty of Engineering, Department of Chemical Engineering, University of Ege, 35100 Bornova, Izmir, Turkey
Reiner Reimert and Hans Schulz Engler-Bunte Institute, Karlsruhe University, D-7500 Karlsruhe, Germany Received January 14, 1998. Revised Manuscript Received June 22, 1998
A fixed bed reactor under argon flow was used to pyrolyze small samples of low- (LDPE) and high-density (HDPE) polyethylene. A special gas-phase sampling technique was used to determine the composition of products eluted from the reactor as a function of temperature and time. Capillary gas chromatography was used to determine the total volatile product evolution rate. The maximum volatile product evolution temperature was 425 °C for LDPE and 430 °C for HDPE. The distribution of n-paraffins and 1-olefins was determined. The hydrocarbon fraction from LDPE contained 47.2 wt % n-paraffins at the maximum product evolution temperature; the weight percentages were 6.6 gaseous n-paraffins, 9.1 C5-C9, 23.7 C10-C15, and 60.6 C16. The yield and distribution of hydrocarbons from pyrolysis of HDPE at the maximum product evolution temperature of 430 °C were similar to that found for LDPE. n-Paraffins constituted 48.8 wt %, of which 5.5 wt % were gaseous n-paraffins, 9.4 wt % C5-C9, 23.5 wt % C10-C15, and 61.6 wt % C16+. 1-Olefin products were also grouped by carbon number. Pyrolysis of LDPE produced 14 wt % of 1-olefins at the maximum product evolution temperature; weight percentages were 20.7 1-olefins gases, 22.8 C5-C9, 26.4 C10-C15, and the 30.1 C16+. Pyrolysis of HDPE produced 14.4 wt % of 1-olefins, of which 33.3 wt % were 1-olefin gases, 40.9 wt % C5-C9, 17.3 wt % C10-C15, and 8.5 wt % C16+.
Introduction Disposal of waste polymers is a major environmental problem. Because they are not easily biodegraded and because of their low weight-to-volume ratios, plastics are not appropriate candidates for landfill.1,2 Burning plastics contributes to air pollution. Some plastic wastes can be recycled or used in secondary and tertiary conversion schemes. Secondary recycling is generally defined as reconversion of plastic into monomers and/ or other direct building blocks, while tertiary recycling is defined as conversion to fuels or chemicals.4 Waste commodity plastics such as low- (LPDE) and high- (HDPE) density polyethylene, poly(ethylene terephthalate) (PET), polystyrene (PS), and poly(vinyl chloride) (PVC) have a high hydrogen-to-carbon ratios and molecular chain structures suitable for direct liquefaction, which is a potential disposal option.5-8 The products produced could be upgraded to transportation fuels. Properties of product gases and oils from the thermal decomposition of a variety of polymeric materials have been investigated. Thermal decomposition of polypro* To whom correspondence should be addressed. E-mail: ballice@ eng.ege.edu.tr. (1) Ding, W.; Liang, J.; Anderson, L. L. Fuel Process. Technol. 1997, 51, 47.
Table 1. Properties of the Used Polyethylene Materials density, g cm-3 melt flow index, g/10 min softening point, °C melting point, °C
HDPE
LDPE
0.964 0.35 128 134
0.30 0.30 98 105
pylene (PP) at 360-400 °C ranged from C1 to C8 hydrocarbons.9 It was observed that chain scission was primarily a random process for poly-(R-olefins) but not for PS. Trimers were the upper size limit for PS while the cracked products for polyethylene had size limit.9 In the present study, the temperature-programmed pyrolysis of LDPE and HDPE was investigated using a new, highly efficient technique. In particular, the temperature that produced the maximum evolution rate (2) Ding, W. B.; Tuntawiroon, W.; Liang, J.; Anderson, L. L. Fuel Process. Technol. 1996, 49, 63. (3) Zmierczak, W.; Xiao, X.; Shabtai J. Fuel Process. Technol. 1997, 51, 1996, 47, 177. (4) Liu, K.; Henk, L. C. Fuel Process. Technol. 1996, 49, 1. (5) Murty, M. V. S.; Rangarajan, P.; Grulke, E. A.; Bhattacharyya, D. Fuel Process. Technol. 1997, 49, 75. (6) Ochoa, R.; Woert, H. V.; Lee, W. H.; Subramanian, R.; Kugler, E.; Eklund, P. C. Fuel Process. Technol. 1996, 49, 119. (7) Feng, Z.; Zhao, J.; Rockwell, J.; Baily, D.; Huffman, G. Fuel Process. Technol. 1996, 49, 17. (8) Luo, M.; Curtis, C. Fuel Process. Technol. 1996, 49, 91. (9) Luo, M.; Curtis, C. Fuel Process. Technol. 1997, 49, 177.
S0887-0624(98)00004-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/14/1998
926 Energy & Fuels, Vol. 12, No. 5, 1998
Ballice et al.
Figure 1. Thermogravimetric analysis curves of LDPE and HDPE.
Figure 3. Gas chromatograms of organic products at maximum evolution temperatures during pyrolysis of HDPE. Table 2. Conditions for Gas Chromatography column stationary phase film thickness carrier gas detector detector temperature injector temperature temperature program
Figure 2. Gas chromatograms of organic products at maximum evolution temperatures during pyrolysis of LDPE.
for volatile products has been determined, and the organic product composition has been characterized in terms of n-paraffin and 1-olefin distributions as a function temperature. The degree of recovery of carbon in the plastic feed material in the form of aliphatic hydrocarbons has been determined. Experimental Section Samples. Beads of HDPE and LDPE were obtained from The Petkim Petrochemical Company, I˙ zmir, Turkey. The beads were ground in an air-cooled jaw mill and sieved to obtain the desired
