Article pubs.acs.org/EF
Temperature Effects on the Yield of Gaseous Olefins from Waste Polyethylene via Flash Pyrolysis Pravin Kannan,* Ahmed Al Shoaibi, and C. Srinivasakannan Department of Chemical Engineering, The Petroleum Institute, Post Office Box 2533, Abu Dhabi, United Arab Emirates ABSTRACT: Thermal pyrolysis has earned a significant reputation in the recovery of valuable chemicals, especially petrochemical-based fuels from various materials, including biomass and plastics. Of many available technologies, flash pyrolysis was preferred in the present work, owing to its minimal heat- and mass-transfer limitations and better control on process conditions. The focus of the work is toward efficient conversion of waste plastics into gaseous products, consisting primarily of alkanes, alkenes, and aromatics. Lab-scale experiments were performed using a flash microreactor to determine a favorable reaction temperature that would result in the maximum yield of olefins, primarily the monomer ethylene.
1. INTRODUCTION Reuse of mixed waste plastics was considered only for lowgrade secondary applications before development of recycling methods that produce basic petrochemicals used as feedstock to make virgin plastics.1 Cracking of polyethylene (PE) into either its constituent monomer or other low-molecular-weight hydrocarbons has become vital because of the increased amounts of PE wastes in the present world. Pyrolysis and/or gasification of PE serve as an appropriate tool for simultaneous recovery of material and energy, thereby reducing the load on waste disposal.2 Pyrolysis of plastics results in degradation of the material, wherein different bonds (C−C backbone and side chains) of the polymer break down to produce lower molecular weight oligomers and monomers. Products whose boiling points are lower than the degradation temperature may vaporize and evolve in the vapor phase, while others may remain as a hydrocarbon oil/liquid. Sometimes the resulting vapors are condensed to produce an oil/wax hydrocarbon product with a high degree of purity and can be refined at the petroleum refinery to produce a range of petrochemical products, including plastic.3 In thermal degradation of polymers, two key process parameters play a role that define the product yield and composition, which is the heating rate and temperature of pyrolysis. As seen from Figure 1, the pyrolysis product varies from gas or liquid or char or mixtures depending upon the type of pyrolysis mode. Flash gas involving high heating rates of about 1000 °C/s and high temperatures rapidly breaks down the oil products and, thus, yields mainly gaseous products. A thorough literature review reveals that the studies pertaining to pyrolysis of PE are performed in a variety of reactors, which include fixed bed,4 fluidized bed,5,6 ultrafast,7 free-fall reactor (FFR),8 and autoclave,9 under different process conditions. Although pyrolysis can be performed under various modes, the ease in operation and commercial scale up favor a fluidized-bed operation evidenced from the majority of the work using a fluidized-bed pyrolyzer. Reddy et al.2 reviewed the published works on PE thermal pyrolysis using a fluidized bed and found that majority of the studies use a bubbling fluidized bed because of the ease in design and operation. Upon comparison of a few similar studies, certain conclusions were © 2014 American Chemical Society
Figure 1. Various types of pyrolyses and their products.
drawn and have been outlined below: (1) As the pyrolysis temperature increases, the gas yield increases with a simultaneous decrease in the oil and wax content. The trend is markedly significant at temperatures above 650 °C and until ca. 750 °C. (2) The residence time has an effect only at low and moderate temperatures (99 95 92.9 62.2 58.6 >90 93 37.0
= = = = = = = =
1000 °C; reaction time < 250 ms; 0.3 mg of LDPE 900 °C; reaction time = 750 ms; 10 μg of LDPE 797 °C; residence time = 400 ms; 0.45 g/s LDPE 790 °C; residence time = 500 ms 800 °C 865 °C; residence time = 600 ms; 21 g/min LDPE 750 °C; residence time = 5000 ms 875 °C; 2 g/min LDPE
reference this work 10 10 5 12 7 13 11
(12) Kaminsky, W.; Schlesselmann, B.; Simon, C. Olefins from polyolefins and mixed plastics by pyrolysis. J. Anal. Appl. Pyrolysis 1995, 32, 19−27. (13) Westerhout, R. W. J.; Waanders, J.; Kuipers, J. A. M.; van Swaaij, W. P. M. Recycling of polyethene and polypropene in a novel benchscale rotating cone reactor by high-temperature pyrolysis. Ind. Eng. Chem. Res. 1998, 37 (6), 2293−2300.
simultaneous increase in the total gas yield. At a temperature of around 950−1000 °C, it is possible to recover up to 48% ethylene monomer with a gas yield of >99%. The combination of an ultrafast heating rate, a high temperature, and a minimal vapor residence time prohibits progress of secondary reactions, thereby yielding high yields of ethylene. A near 50% conversion of PE into its highly reactive, base monomer along with smaller proportions of low carbon “lene” and “ene” compounds signify the importance of fast pyrolysis.
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process conditions T T T T T T T T
AUTHOR INFORMATION
Corresponding Author
*Telephone: +971-26075198. Fax: +971−26075200. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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REFERENCES
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dx.doi.org/10.1021/ef500516n | Energy Fuels 2014, 28, 3363−3366
