Abstract
The two-stage pyrolysis-catalysis of high density polyethylene has been investigated with pyrolysis of the plastic in the first stage followed by catalysis of the evolved hydrocarbon pyrolysis gases in the second stage using solid acid catalysts to produce gasoline range hydrocarbon oil (C8–C12). The catalytic process involved staged catalysis, where a mesoporous catalyst was layered on top of a microporous catalyst with the aim of maximising the conversion of the waste plastic to gasoline range hydrocarbons. The catalysts used were mesoporous MCM-41 followed by microporous ZSM-5, and different MCM-41:zeolite ZSM-5 catalyst ratios were investigated. The MCM-41 and zeolite ZSM-5 were also used alone for comparison. The results showed that using the staged catalysis a high yield of oil product (83.15wt.%) was obtained from high density polyethylene at a MCM-41:ZSM-5 ratio of 1:1 in the staged pyrolysis-catalysis process. The main gases produced were C2 (mainly ethene), C3 (mainly propene), and C4 (mainly butene and butadiene) gases. In addition, the oil product was highly aromatic (95.85wt.% of oil) consisting of 97.72wt.% of gasoline range hydrocarbons.In addition, pyrolysis-staged catalysis using a 1:1 ratio of MCM-41: zeolite ZSM-5 was investigated for the pyrolysis–catalysis of several real-world waste plastic samples from various industrial sectors. The real world samples were, agricultural waste plastics, building reconstruction plastics, mineral water container plastics and household food packaging waste plastics. The results showed that effective conversion of the real-world waste plastics could be achieved with significant concentrations of gasoline range hydrocarbons obtained.
Highlights
Plastics are high demand materials, employed in a wide range of applications, from household packaging, agricultural, automotive, to building construction, due to their lightweight characteristics, high chemical stability and low degradability [1]
This paper reports on the catalytic pyrolysis of waste plastic in the form of high density polyethylene in a two-stage pyrolysiscatalysis system
The purpose of using two catalysts staged one after the other in layers was; firstly, to induce the pre-cracking of the large molecules of pure high density polyethylene (HDPE) produced from pyrolysis into smaller molecules via the larger mesopores of the MCM-41 catalyst; secondly, the resultant smaller fractions produced from the MCM-41 catalyst layer may enter the smaller micropores of the ZSM-5 catalyst for futher cracking, mainly into a gasoline range hydrocarbon fraction [17]
Summary
Plastics are high demand materials, employed in a wide range of applications, from household packaging, agricultural, automotive, to building construction, due to their lightweight characteristics, high chemical stability and low degradability [1]. Much of the plastic will end up in the waste stream and in the EU more than 25 million tonnes of waste plastics are generated each year. There is significant export of waste plastics, with a reported 3.4 million tonnes of waste plastic exported out of the EU each year [4]. The process options for recycling waste plastics in the EU are dominated by mechanical recycling which involves for example, sorting, shredding, washing drying and pelletising of the plastic producing a recyclate material. There is growing interest in the pyrolysis of waste plastics, where the plastic is thermally degraded at moderate temperature (∼500 ◦C) in the absence of air to produce shorter molecular chains and low molecular weight molecules to produce an oil for potential use as a liquid fuel as reported in several recent reviews [5,6,7,8]
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