FEEDSTOCK RECYCLING OF AGRICULTURE FILM WASTES BY CATALYTIC CRACKING
Abstract
Feedstock recycling by catalytic cracking of a real plastic film waste from Almeria greenhouses (Spain) towards valuable hydrocarbon mixtures has been studied over several acid catalysts. The plastic film waste was mostly made up of ambient degraded low-density polyethylene (LDPE) and ethylene-vinyl acetate copolymer (EVA), the vinyl acetate content being around 4 wt %. Nanocrystalline HZSM-5 zeolite (crystal size 60 nm) was the only catalyst capable of degrading completely the refuse at 420ºC despite using a very small amount of catalyst (plastic / catalyst mass ratio of 50). However, mesoporous catalysts (Al-SBA-15 and Al-MCM-41), unlike it occurred with virgin LDPE, showed fairly close conversions to that of thermal cracking. Nanocrystalline HZSM-5 zeolite led to 60 wt % selectivity towards C1 - C5 hydrocarbons, mostly valuable C3 - C5 olefins, what would improve the profitability of a future industrial recycling process. The remarkable performance of nanocrystalline HZSM-5 zeolite was ascribed to its high content of strong external acid sites due to its nanometer dimension, which are very active for the cracking of bulky macromolecules. Hence, nanocrystalline HZSM-5 can be regarded as a promising catalyst for a feasible feedstock recycling process by catalytic cracking. [...]
1. Introduction
Plastic wastes have become a subject of increasing worldwide environmental concern. According to 2000 data [1], the total plastic consumption in Western Europe increased to 35.8 million tonnes (Mt). The plastic wastes collected this same year amounted to 18.8 Mt, the great majority ending up their lifetime as a part of the municipal solid waste stream (MSW). In 1994, the European Union, aware of this problem, launched the 94/62/CE directive on waste packaging that established minimum rates of recycling per material (15 %). Since then, the percentages of recycling and energy recovery have increased to 12.6 % and 20.2 % respectively, but the preferred management strategies of plastic wastes is still landfilling (64 %).
The development of recycling procedures for polyolefins such as low-density polyethylene (LDPE), high-density polyethylene (HDPE) and polypropylene (PP), is highly encouraged since they account for roughly 60 % of the total plastic wastes [2]. In practice, these polyolefin refuse can be recycled either mechanically towards a new plastic with similar / lower applications, or chemically to obtaining chemical raw materials or fuels. However, mechanical recycling should be regarded as an intermediate procedure due to their inherent bounds (progressive loss of plastic properties through melting, saturation of potential end-markets, etc.) [3]. Thus, sooner or latter, mechanically recycled plastics should be treated by other procedures such as feedstock recycling, energy recovery or landfilling. Keywords: Feedstock recycling, plastic wastes, catalytic cracking, nanocrystalline ZSM-5 zeolite
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